μM PO4 Research Articles

Phytoplankton, protozooplankton and nutrient dynamics in the Bornholm Basin (Baltic Sea) in 2002–2003 during the German GLOBEC Project

From March 2002 to until April 2003 we investigated the seasonal nutrient and phytoplankton dynamics in the central Bornholm Basin (Baltic Sea) within the framework of the German GLOBEC Project. We choose a nested approach consisting of vertical fluorescence profiles, phytoplankton counts and nutrient analyses. The Fluoroprobe (MultiProbe, BBE Moldaenke) is capable of distinguishing four algal groups (Cryptophyceae, Cyanophyceae, Chlorophyceae, Bacillariophyceae + Dinophyceae). Winter nutrient concentrations were about 5 μM NO3 and 0.5 μM PO4 in the central Basin. The spring phytoplankton bloom was dominated by the diatom Skeletonema sp. and reached a maximum of about 270 μg C/l before the onset of the seasonal stratification. Protozooplankton was dominated by the Mesodinium rubrum (a phototrophic ciliate = Myrionecta rubra) and reached a maximum biomass of about 200–300 μg C/l about 2 weeks after the demise of the diatom spring bloom. During summer, the water column was stratified and a subsurface maximum developed near the thermocline consisting of Bacillariophyceae, Cryptophycea and other phototrophic flagellates. Phytoplankton and protozooplankton biomass was generally low. Nutrient concentrations point towards a nitrogen limitation during this period. The stratification period ended during September and surface nutrient concentrations increased again. Protozooplankton reached a second maximum during September. With the Fluoroprobe small scale structures in the plankton community could be detected like a subsurface Cryptophyceae maximum near the thermocline that however, could not be confirmed by cell counts. The chlorophyll a estimate of the Fluoroprobe was in good agreement with the phytoplankton biomass estimated from counts. We conclude that only by combining modern sensing technology with microscopy, the small-scale dynamics and taxonomic spectrum of the plankton can be fully captured.

Seasonal dynamics of inorganic nutrients and phytoplankton biomass in the NW Alboran Sea

The seasonal dynamics of inorganic nutrients and phytoplankton biomass (chlorophyll a), and its relation with hydrological features, was studied in the NW Alboran Sea during four cruises conducted in February, April, July and October 2002. In the upper layers, the seasonal pattern of nutrient concentrations and their molar ratios (N:Si:P) was greatly influenced by hydrological conditions. The higher nutrient concentrations were observed during the spring cruise (2.54 μM NO 3 −, 0.21 μM PO 4 3− and 1.55 μM Si(OH) 4, on average), coinciding with the increase of salinity due to upwelling induced by westerlies. The lowest nutrient concentrations were observed during summer (<0.54 μM NO 3 −, 0.13 μM PO 4 3− and 0.75 μM Si(OH) 4, on average), when the lower salinities were detected. Nutrient molar ratios (N:Si:P) followed the same seasonal pattern as nutrient distribution. During all the cruises, the ratio N:P in the top 20 m was lower than 16:1, indicating a NO 3 − deficiency relative to PO 4 3−. The N:P ratio increased with depth, reaching values higher than 16:1 in the deeper layers (200–300 m). The N:Si ratio in the top 20 m was lower than 1:1, excepting during spring when N:Si ratios higher than 1:1 were observed in some stations due to the upwelling event. The N:Si ratio increased with depth, showing a maximum at 50–100 m (>1.5:1), which indicates a shift towards Si-deficiency in these layers. The Si:P ratio was much lower than 16:1 throughout the water column during the four cruises. In general, the spatial and seasonal variation of phytoplankton biomass showed a strong coupling with hydrological and chemical fields. The higher chlorophyll a concentrations at the depth of the chlorophyll maximum were found in April (2.57 mg m −3 on average), while the lowest phytoplankton biomass corresponded to the winter cruise (0.74 mg m −3 on average). The low nitrate concentrations together with the low N:P ratios found in the upper layers (top 20 m) during the winter, summer and autumn cruises suggest that N-limitation could occur in these layers during great part of the year. However, N-limitation during the spring cruise was temporally overcome by nutrient enrichment caused by an intense wind-driven upwelling event.

Effects of sediment and nutrient enrichment on water quality in the Archipelago Sea, northern Baltic: An enclosure experiment in shallow water

The effects of bottom sediment and nutrient enrichment on water quality were studied in an enclosure experiment in the Archipelago Sea, northern Baltic. The three-week experiment was conducted in a small and shallow bay, where the organic content of the sediment is low. The enclosures were large (diameter 3.6 m, depth 3.5 m), and reached from the surface to the bottom. Some of the enclosures included the natural sediment, some had a plastic bottom without contact with the sediment. Concentrations of nutrients and chlorophyll a and physical variables in the water column, concentrations of inorganic nutrients in the pore water of the sediment, and numbers and biomasses of the benthic macrofauna were measured. Both the presence of sediment and nutrient enrichment significantly increased the concentrations of total nitrogen, total phosphorus and chlorophyll a in the water column. The concentration of chlorophyll a doubled in the sediment-bottomed enclosures without nutrient enrichment; the increase was similar to that in the plastic-bottomed enclosures with nutrient enrichment (7.2 μM NH 4 + and 0.46 μM PO 4 3− during the three-week study). The concentration of silica doubled or tripled in the sediment-bottomed enclosures. No shortage of oxygen was found in the water column during the experiment. The results show that sediment with a low content of organic matter may serve as an important source of nutrients in shallow and littoral oxic waters and may be important in sustaining their eutrophic state during the productive season. It is suggested that an important part of the nutrients released from this erosion bottom had originated from the surface layer of the sediment, which had been sedimented and transported to the area quite recently. The results indicate that it is important to include sediment in mesocosm studies dealing with nutrient dynamics, especially in shallow waters.

Ecophysiology of the marine cyanobacterium, Lyngbya majuscula (Oscillatoriaceae) in Moreton Bay, Australia

Large blooms of the marine cyanobacterium Lyngbya majuscula in Moreton Bay, Australia (27°05′S, 153°08′E) have been re-occurring for several years. A bloom was studied in Deception Bay (Northern Moreton Bay) in detail over the period January–March 2000. In situ data loggers and field sampling characterised various environmental parameters before and during the L. majuscula bloom. Various ecophysiological experiments were conducted on L. majuscula collected in the field and transported to the laboratory, including short-term (2 h) 14C incorporation rates and long-term (7 days) pulse amplitude modulated (PAM) fluorometry assessments of photosynthetic capacity. The effects of L. majuscula on various seagrasses in the bloom region were also assessed with repeated biomass sampling. The bloom commenced in January 2000 following usual December rainfall events, water temperatures in excess of 24 °C and high light conditions. This bloom expanded rapidly from 0 to a maximum extent of 8 km 2 over 55 days with an average biomass of 210 g dw −1 m −2 in late February, followed by a rapid decline in early April. Seagrass biomass, especially Syringodium isoetifolium, was found to decline in areas of dense L. majuscula accumulation. Dissolved and total nutrient concentrations did not differ significantly ( P > 0.05) preceding or during the bloom. However, water samples from creeks discharging into the study region indicated elevated concentrations of total iron (2.7–80.6 μM) and dissolved organic carbon (2.5–24.7 mg L −1), associated with low pH values (3.8–6.7). 14C incorporation rates by L. majuscula were significantly ( P < 0.05) elevated by additions of iron (5 μM Fe), an organic chelator, ethylenediaminetetra-acetic acid (5 μM EDTA) and phosphorus (5 μM PO 4 −3). Photosynthetic capacity measured with PAM fluorometry was also stimulated by various nutrient additions, but not significantly ( P > 0.05). These results suggest that the L. majuscula bloom may have been stimulated by bioavailable iron, perhaps complexed by dissolved organic carbon. The rapid bloom expansion observed may then have been sustained by additional inputs of nutrients (N and P) and iron through sediment efflux, stimulated by redox changes due to decomposing L. majuscula mats.

Potential P limitation leads to excess N in the pearl river estuarine coastal plume

There is excess nitrate (NO3) in the Pearl River coastal plume in the southern waters of Hong Kong in summer. We hypothesize that phosphorus (P) limitation controls the utilization of excess NO3 due to the high N:P ratio in the Pearl River. To test this hypothesis, we conducted two 1-day cruises on July 13 and 19, 2000 to examine the response of the phytoplankton to P additions with respect to changes in biomass, uptake of nutrients and nutrient uptake ratios using a batch incubation of natural water samples collected from the Pearl River estuary and adjacent coastal waters. At a station (E1, salinity=5) in the Pearl River estuary, the N/P ratio at the surface was 46:1, (64μM DIN: 1.3μM PO4) and decreased to 24:1 (12μM DIN: 0.5μM PO4) downstream at a station (Stn 26, salinity=26) in the coastal plume south of Hong Kong. Without a P addition, NO3 in the water samples collected at E1 could not be depleted during a 9 day incubation (∼20 μM NO3 remaining). With a P addition, NO3 disappeared completely on day 6 with the depletion of the added PO4 (2–3μM). This was also true for a station, E4 (salinity=15) further downstream, but within the estuary. At Stn 26, in the coastal plume south of Hong Kong, NO3 (∼11.5μM) was eventually depleted without the addition of PO4, but it took 8 days instead of 5 days for Stn E4. The uptake ratio of dissolved inorganic nitrogen (DIN) to PO4, without a P addition was 51:1, 43:1 and 46:1 for Stns E1, E4 and 26, respectively. With a P addition, the DIN/PO4 uptake ratio decreased to 20:1, 14:1 and 12:1, respectively, for the 3 stations. These results clearly indicate potential P limitation to utilization of NO3 in the Pearl River estuary, resulting in excess NO3 in waters of the coastal plume downstream of the estuary, some of which would eventually be transported offshore. High uptake ratios of N:P without a P addition (43N:1P) suggest that phytoplankton have a nitrogen uptake capacity in excess of the Redfield ratio of 16N: 1P by 2.5–3 times. The value of 2.5–3 times was likely a maximum that should have contained a contribution of P released from desorption of P from sediments or from regeneration by zooplankton grazing and bacterial activity during the incubation of natural water samples. Without a P addition, however, phytoplankton biomass did not increase. This means that P turnover rates or regeneration may allow phytoplankton to take up additional N in excess of the Redfield ratio and store it, but without increasing the algal biomass. Therefore, high ambient N:P ratios in excess of the Redfield ratio do indicate potential P limitation to phytoplankton biomass in this estuarine coastal ecosystem.

Growth, nutrient storage, and release of dissolved organic nitrogen by Enteromorpha intestinalis in response to pulses of nitrogen and phosphorus

Enteromorpha intestinalis was subject to nitrogen (N) and phosphorus (P) enrichment supplied in four weekly pulses in a 10:1 molar ratio with six concentrations ranging from no addition to 1000 μM NO 3+100 μM PO 4. The alga reduced inorganic N to very low concentrations (<3.5 μM) across all nutrient additions while a larger proportion of the added P remained in the water at the end of the experiment (up to 20 μM in highest addition). Growth of E. intestinalis increased in proportion to enrichment across the lower four of the six treatments; range of growth was −6 to 60% change from initial wet wt. N concentration in the tissue decreased at the four lower loading rates due to dilution by growth, but increased greatly (from an initial of 2.7–5% dry wt.) in the highest loading rate partially due to lack of growth in this treatment. In contrast, the mass of N in the tissue increased linearly (slope=0.012, r 2=0.779, P=0.0001) over all treatments, showing that uptake and storage were not always coupled to growth. Patterns of tissue P concentration and accumulation were similar to N. Significant amounts of dissolved organic N accumulated in the water, especially in the highest N pulses. Net retention of N in algal tissue ranged from 73 to 98% while retention of P ranged from 79 to 88%; the % lowest retention occurred when nutrient pulses were either very high or low. We hypothesize that this opportunistic, bloom-forming alga is adapted to pulses of nutrients, such that, when pulses are of very high concentration, E. intestinalis can delay growth in favor of saving energy to maximize nutrient uptake and storage.

Nutrient uptake by coral-reef microatolls

We measured rates of ammonium and phosphate uptake into 12 microatolls at One Tree reef lagoon, Australia, on 14 occasions over a range of seasonal and meteorological conditions from 1993 to 1995. Nine of the microatolls were spiked with ammonia and/or phosphate every low tide (2.5–4 h) during low- (15-month) and high- (13-month) loading periods. The remaining three microatolls that were not spiked with nutrient served as reference conditions. Ammonium concentrations were elevated from an average background of 0.7 to 11 µM NH4 during the low-loading period and 36 µM NH4 during the high-loading period, resulting in a loading to the benthos of 3.5 and 18 mmol NH4 m–2 low tide–1. Phosphate concentrations were elevated from an average background of 0.2 to 2.3 and 5.1 µM PO4 during low- and high-loading periods, respectively, resulting in a loading of 0.66 and 3.9 mmol PO4 m–2 low tide–1. Ammonium and phosphate concentrations decreased significantly over low-tides, and uptake rates were proportional to concentrations (first-order). The average uptake-rate constant, S (m s–1), for ammonium and phosphate did not differ between the two loading periods but was highly variable. Averaged over both loading periods, S for ammonium was 129±74×10–6 m s–1 and S for phosphate was 67±39×10–6 m s–1. At background nutrient concentrations, estimated nutrient-uptake rates were 7.8 mmol NH4 m–2 day–1 and 1.2 mmol PO4 m–2 day–1. Excretion rates—calculated from the mean difference in uptake rates measured in reference and nutrient-enriched microatolls—were estimated to be 4.3 mmol NH4 m–2 day–1 and 0.9 mmol PO4 m–2 day–1. We reason and suggest that nutrient uptake rates in these microatolls were close to mass-transfer limited rates. We conclude that nutrient uptake into coral reefs can be highly dynamic, varying 10-fold spatially and temporally.

HIGH ENCYSTMENT SUCCESS OF THE DINOFLAGELLATESCRIPPSIELLACF.LACHRYMOSAIN CULTURE EXPERIMENTS1

Close to 100% encystment efficiency and a yield above 105cysts·mL−1were routinely achieved in full strength f/2 medium‐based batch cultures (883 μM NO3−and 36 μM PO4−3) of the marine dinoflagellateScrippsiellacf.lachrymosaLewis. Increases in cell density led to nutrient depletion in this enriched medium, which was the most likely cause for initiation of cyst formation. Lowering the concentration of either nutrient to 1/10 the initial levels decreased the encystment efficiency, whereas use of ammonium as the N source resulted in both low cell yield and low encystment efficiency. The mandatory dormancy period was ca. 60 days and was not affected by cold dark storage of the cysts. Cysts produced in the initial phase of sexual reproduction were relatively large (length 47 μm, width 31 μm) with a heavy calcareous cover. Cysts produced thereafter lacked apparent calcareous cover and were smaller (length 29 μm, width 19 μm). The decrease of cyst volume (by a factor of 0.24–0.4) suggested strong resource limitation during the course of encystment. However, after the mandatory dormancy period, germination success of the smaller cysts was higher (80%), compared with the larger cysts that had been produced initially (50%). Germling survival (74%) was independent of cyst type but was enhanced by higher nutrient concentration during incubation. The ratio of initial nutrient concentration in the medium to the cyst yield was used as a proxy to estimate the cellular nutrient quota. The conservative estimates of 9 pmol N·cyst−1and 0.4 pmol P·cyst−1obtained in this manner are at the low end of the range of previous published estimates for other dinoflagellate cysts. Given the high encystment observed in laboratory experiments, we have no reason to assume an inherently lower encystment success in dinoflagellate field populations. Our results do not challenge the low nutrient paradigm for dinoflagellate sexuality. We believe that the high encystment success and cyst yield of this particular species is at least partly due to its ability to achieve very high cell densities in cultures, which evidently leads to nutrient depletion even in f/2 medium.

The importance of mineralization based on sulfate reduction for nutrient regeneration in tropical seagrass sediments

Nutrient dynamics (nitrogen and phosphate), sediment organic matter mineralization (sulfate reduction rates) and stable carbon isotope composition in two tropical seagrass sediments were studied at a intertidal sandflat of Ban Pak Klok, Thailand. The seagrass beds were composed of meadows of Cymodocea rotundata (Ehrenb. and Hempr.) Aschers and Thalassia hemprichii (Ehrenb.) Aschers. The nutrient concentrations in porewaters were low (<1 μM NO 2 −+NO 3 −, 30–70 μM NH 4 + and 2–5 μM PO 4 3−) in the low-organic sediments (<0.6% DW POC and <0.03% DW PON). Sulfate reduction rates were significantly higher in the C. rotundata sediments (6.6 mmol m −2 per day), whereas there was no difference between rates in the T. hemprichii and bare sediments (2.1 and 2.2 mmol m −2 per day). The depth distribution of sulfate reduction rates in C. rotundata sediments was positively correlated with the below-ground biomass suggesting that the enhanced anaerobic activity was due to supply of microbial substrates from the seagrasses. The stable isotopic composition of the sediment bacteria resembled the seagrasses suggesting that the bacteria used organic matter derived from the seagrasses. A two-compartment experiment showed that both seagrasses released organic compounds from the roots (7.0 and 5.4% of photosynthetic rate for C. rotundata and T. hemprichii, respectively), and that the loss was sufficient to support the measured sulfate reduction rates. The contribution of sulfate reduction to nutrient availability was, however, low supporting only 6–22% of the nutrient demand, except for PO 4 3− in C. rotundata sediments where 81% of the demand was covered by sulfate reduction. The relatively high nutrient content of the seagrasses (1.5–2.0% DW N and 0.18–0.20% DW P) suggested that their growth was not limited by nutrients. The low contribution of sulfate reduction to nutrient availability thus indicates that other mineralization processes or uptake of nutrients from the water column are important for plant growth.

High temporal resolution field monitoring of phosphate in the River Frome using flow injection with diode array detection

The design and deployment of an in situ flow injection (FI) monitor for high temporal resolution monitoring of phosphate in the River Frome, Dorset, UK, is described. The monitor incorporates solenoid, self-priming micropumps for propulsion, solenoid-operated switching valves for controlling the fluidics and a miniature CCD spectrometer for full spectrum (200–1000 nm) acquisition and operates in a graphical programming environment. A tangential filtration unit is attached to the sample inlet line to remove suspended particulate matter and prevent blockage of the micropumps and valves. Detection (at 710 nm) is based on molybdenum blue chemistry with tin(II) chloride reduction. The detection limit is 0.67 μM PO 4 and the linear range can be adjusted by using different wavelengths for detection. Pump noise is eliminated by subtraction of the signal at a non-absorbing wavelength (447 nm). Data from an intensive (sample every 30 min) field trial on the River Frome performed in October 2000 are presented, and the implications of the data for refining an export coefficient model for phosphorus from the catchment are discussed.

ENCORE: The Effect of Nutrient Enrichment on Coral Reefs. Synthesis of Results and Conclusions

Coral reef degradation resulting from nutrient enrichment of coastal waters is of increasing global concern. Although effects of nutrients on coral reef organisms have been demonstrated in the laboratory, there is little direct evidence of nutrient effects on coral reef biota in situ. The ENCORE experiment investigated responses of coral reef organisms and processes to controlled additions of dissolved inorganic nitrogen (N) and/or phosphorus (P) on an offshore reef (One Tree Island) at the southern end of the Great Barrier Reef, Australia. A multi-disciplinary team assessed a variety of factors focusing on nutrient dynamics and biotic responses. A controlled and replicated experiment was conducted over two years using twelve small patch reefs ponded at low tide by a coral rim. Treatments included three control reefs (no nutrient addition) and three+N reefs (NH 4Cl added), three+P reefs (KH 2PO 4 added), and three+N+P reefs. Nutrients were added as pulses at each low tide ( ca twice per day) by remotely operated units. There were two phases of nutrient additions. During the initial, low-loading phase of the experiment nutrient pulses (mean dose=11.5 μM NH 4 + ; 2.3 μM PO 4 −3) rapidly declined, reaching near-background levels ( mean=0.9 μM NH 4 + ; 0.5 μM PO 4 −3 ) within 2–3 h. A variety of biotic processes, assessed over a year during this initial nutrient loading phase, were not significantly affected, with the exception of coral reproduction, which was affected in all nutrient treatments. In Acropora longicyathus and A. aspera, fewer successfully developed embryos were formed, and in A. longicyathus fertilization rates and lipid levels decreased. In the second, high-loading, phase of ENCORE an increased nutrient dosage (mean dose=36.2 μM NH 4 + ; 5.1 μM PO 4 −3 declining to means of 11.3 μM NH 4 + and 2.4 μM PO 4 −3 at the end of low tide) was used for a further year, and a variety of significant biotic responses occurred. Encrusting algae incorporated virtually none of the added nutrients. Organisms containing endosymbiotic zooxanthellae (corals and giant clams) assimilated dissolved nutrients rapidly and were responsive to added nutrients. Coral mortality, not detected during the initial low-loading phase, became evident with increased nutrient dosage, particularly in Pocillopora damicornis. Nitrogen additions stunted coral growth, and phosphorus additions had a variable effect. Coral calcification rate and linear extension increased in the presence of added phosphorus but skeletal density was reduced, making corals more susceptible to breakage. Settlement of all coral larvae was reduced in nitrogen treatments, yet settlement of larvae from brooded species was enhanced in phosphorus treatments. Recruitment of stomatopods, benthic crustaceans living in coral rubble, was reduced in nitrogen and nitrogen plus phosphorus treatments. Grazing rates and reproductive effort of various fish species were not affected by the nutrient treatments. Microbial nitrogen transformations in sediments were responsive to nutrient loading with nitrogen fixation significantly increased in phosphorus treatments and denitrification increased in all treatments to which nitrogen had been added. Rates of bioerosion and grazing showed no significant effects of added nutrients. ENCORE has shown that reef organisms and processes investigated in situ were impacted by elevated nutrients. Impacts were dependent on dose level, whether nitrogen and/or phosphorus were elevated and were often species-specific. The impacts were generally sub-lethal and subtle and the treated reefs at the end of the experiment were visually similar to control reefs. Rapid nutrient uptake indicates that nutrient concentrations alone are not adequate to assess nutrient condition of reefs. Sensitive and quantifiable biological indicators need to be developed for coral reef ecosystems. The potential bioindicators identified in ENCORE should be tested in future research on coral reef/nutrient interactions. Synergistic and cumulative effects of elevated nutrients and other environmental parameters, comparative studies of intact vs. disturbed reefs, offshore vs. inshore reefs, or the ability of a nutrient-stressed reef to respond to natural disturbances require elucidation. An expanded understanding of coral reef responses to anthropogenic impacts is necessary, particularly regarding the subtle, sub-lethal effects detected in the ENCORE studies.

Effect of nutrient enrichment on growth and photosynthesis of the zooxanthellate coral Stylophora pistillata

The effect of prolonged (9 week) nutrient enrichment on the growth and photosynthetic rates of the zooxanthellate coral Stylophora pistillata was investigated. The main questions were: (1) what is the exposure time needed to induce measurable change in growth rate? (2) which are the concentrations of nitrogen and phosphorus required to cause changes in these rates? (3) what is the recovery potential of the corals after the nutrient stress? For this purpose, three tanks (N, P, NP) were enriched with ammonium (N), phosphorus (P) or both nutrients (NP), respectively. A fourth tank (C) served as a control. The growth of 40 nubbins (10 in each tank) was monitored during four periods: period 1 (nutrient-poor conditions), period 2 (10 μm NH4 and/or 2 μm PO4 enrichment), period 3 (20 μm NH4 and/or 2 μm PO4) and period 4 (nutrient-poor conditions). Period 4 was performed to study the recovery potential of corals after a nutrient stress. During period 1, growth rates remained constant in all tanks. In the P tank, growth rates declined during the two enrichment periods, with a total decrease of 60% by the end of period 3. In the N tank, growth rates remained nearly constant during period 2 but decreased in period 3 (60% decrease). In the NP tank, 50% and 25% decreases were observed during periods 2 and 3. At the end of the recovery period, a regain in growth rate was observed in the N and NP tanks (35 and 30% increase, respectively, compared with the rates measured at the end of period 3) and growth rates returned to 60% of the initial rates. By contrast, in the P tank, there was no regain in growth and a further decrease of 5% was observed. Rates of photosynthesis were often higher during the enriched than the nutrient-poor period (up to 150% increase). Corals with the highest percent increases in maximal gross photosynthetic rate (P g max ) had the smallest decreases in growth rate due to nutrient enrichment. In conclusion, high ammonium (20 μm) and relatively low phosphorus concentrations (2 μm) are required to induce a significant decrease in coral growth rate. The largest reduction was observed with both ammonium and phosphorus enrichment. The decrease in growth rate was rapid following nutrient enrichment, since a 10% decrease or more could be observed after the first week of treatment.

The accumulation and release of polysaccharides by planktonic cells and the subsequent bacterial response during a controlled experiment

Seawater from the northern Adriatic, with low phosphorus (0.03 μM) and nitrogen (1.0 μM NO3 and 1.0 μM NH4) concentration, was incubated for 12 days in 20-l polycarbonate carboys. The addition of a nutrient mixture (0.6 μM PO4, 5.1 μM NO3, 1.8 μM NH4, 10.6 μM SiO2) induced a strong diatom bloom, reaching 25 μg l−1 Chl a. Primary and bacterial production were stimulated by the initial enrichment of nitrogen and phosphorus but ceased when N and P depletion occurred after 4 days. Inorganic N exhaustion resulted in a significant production (and accumulation) of dissolved and particulate carbohydrates. The initial accumulation of carbohydrates (CHO) in the particulate phase was followed 2 days later by a significant release of dissolved CHO. The bacterial response to this organic carbon source, as reflected by glucosidase activity, was probably inhibited by the severe P limitation following the phytoplankton bloom. In the exponential phase, when P concentration was sufficiently high to sustain a significant glucosidase activity, no increase in either dissolved organic carbon or dissolved total CHO was observed. We hypothesise that the periodic accumulation of dissolved organic carbon in the northern Adriatic is due to an excessive nitrogen enrichment followed by a concurrent N and P limitation.

The accumulation and release of polysaccharides by planktonic cells and the subsequent bacterial response during a controlled experiment

Seawater from the northern Adriatic, with low phosphorus (0.03 μM) and nitrogen (1.0 μM NO 3 and 1.0 μM NH 4) concentration, was incubated for 12 days in 20-l polycarbonate carboys. The addition of a nutrient mixture (0.6 μM PO 4, 5.1 μM NO 3, 1.8 μM NH 4, 10.6 μM SiO 2) induced a strong diatom bloom, reaching 25 μg l −1 Chl a. Primary and bacterial production were stimulated by the initial enrichment of nitrogen and phosphorus but ceased when N and P depletion occurred after 4 days. Inorganic N exhaustion resulted in a significant production (and accumulation) of dissolved and particulate carbohydrates. The initial accumulation of carbohydrates (CHO) in the particulate phase was followed 2 days later by a significant release of dissolved CHO. The bacterial response to this organic carbon source, as reflected by glucosidase activity, was probably inhibited by the severe P limitation following the phytoplankton bloom. In the exponential phase, when P concentration was sufficiently high to sustain a significant glucosidase activity, no increase in either dissolved organic carbon or dissolved total CHO was observed. We hypothesise that the periodic accumulation of dissolved organic carbon in the northern Adriatic is due to an excessive nitrogen enrichment followed by a concurrent N and P limitation.

Short‐term responses of coral reef microphytobenthic communities to inorganic nutrient loading

The responses of coral reef flat microphytobenthos to short‐term exposure to elevated levels of inorganic nitrogen (N) and phosphorus (P) were investigated in 1994 and 1995. Sand samples collected from the reef flat were maintained over 7 d in triplicate cultures with N‐enriched (100 µM NO3), P‐enriched (10 µM PO4), and ambient seawater. A fourth experiment used a treatment of combined N and P enrichment. The sediment samples were assessed for chlorophyll a (Chl a) content and photosynthesis‐irradiance (P‐I) responses. P‐I curves, constructed from area‐and Chl a‐specific metabolic rates, showed consistently higher maximal rates in the nutrient enriched samples. Sediments exposed to enhanced levels of N exhibited the highest Chl a content while both N‐ and P‐enriched samples showed increased photosynthetic yield. Very little depletion of nutrients in the water column was detected over time in the batch cultures except in the N:P‐enriched treatments where nutrient values dropped to near‐ambient levels. Results from these experiments point to N and P colimitation in tropical carbonate sediments.

Plant availability of selenite and selenate as influenced by the competing ions phosphate and sulfate

The relative plant availability of selenate versus selenite depends on the concentrations of competing ions, specifically sulfate and phosphate, respectively. In solution culture, the concentration of phosphate is typically 100- to 1000-fold greater than in soil solution, an artifact that could lead to underestimation of the phytoavailability of selenite. A nutrient solution study was conducted to compare the availability of selenite and selenate to perennial ryegrass (Lolium perenne L. cv. Evening Shade) and strawberry clover (Trifolium fragiferrum L. cv. O'Conner) at basal concentrations of SO4 (0.5 mM) and PO4 (2 μM) similar to those found in soil solution. Concentrations up to 5 mM SO4 and 200 μM PO4 allowed quantitative comparison of the efficacy of the competing ions. In both species, selenite was more phytotoxic than selenate, especially for shoot growth. Selenate was less toxic, and tended to preferentially inhibit root growth. Translocation percentages were much higher with selenate (≥84%) than with selenite (≤47%). A 10-fold increase in sulfate decreased uptake from selenate by >90% in both species. In ryegrass, 10-fold increases in phosphate caused 30% to 50% decreases in Se accumulation from selenite, but in clover such decreases only occurred in the roots. Sulfate-selenate antagonisms were thus stronger than phosphate-selenite antagonisms. Nonetheless, conventional nutrient solutions with millimolar phosphate will significantly underestimate Se availability from selenite, and moderate levels of sulfate salinity can inhibit selenate uptake sufficiently to reverse the apparent relative availability of the two forms of Se.

PH-altered interaction of aluminium and fluoride on nutrient uptake, photosynthesis and other variables of Chlorella vulgaris

This study presents information on the pH-induced toxicity of AlCl 3, AlF 3, NaF and AlCl 3+NaF on growth, nutrient uptake, photosynthesis, photosynthetic electron transport, enzymes of nitrogen and phosphorus metabolism and ATPase activity of Chlorella vulgaris. Both PS I and PS II were severely inhibited by the combination of AlCl 3+NaF followed by AlF 3, AlCl 3 and NaF. Inhibition of acid phosphatase activity was more pronounced than alkaline phosphatase. In general, toxicity of all the test chemicals increased with a downshift in pH. The interaction of AlCl 3 and NaF produced additive effect at pH 6.8, and synergistic at pH 6.0 and 4.5. The combination of AlCl 3+NaF did not produce significant difference in the activity of Mg 2+ and Ca 2+-ATPase in the medium supplemented with 60 and 100 μM PO 4 3− at pH 6.8. Compared to 60 μM PO 4 3−, the test organism grown in 100 μM PO 4 3− containing medium depicted significant increase in Mg 2+ and Ca 2+-ATPase activity at pH 6.0 and 4.5. Infra-red spectra revealed the formation of AlF 4 − at pH 6.0 and 4.5 in AlCl 3+NaF supplemented medium as well as in the algal cells. Aluminium uptake from the medium containing AlCl 3 decreased in the presence of PO 4 3−. This study suggests that phosphate restricts the entry of Al into the cell, while fluoride promotes it. AlF 4 − toxicity could be due to its interference with phosphate binding site of ATPase thereby arresting the release of energy.

Effects of pH and Phosphate Competition on the Transport of Arsenate

The effects of pH and phosphate (PO 4 ) competition on arsenate (AsO 4 ) sorption by soils and aquifer materials have been previously investigated in batch studies, however, little has been reported on processes that influence AsO 4 transport through porous media. Consequently, the objective of this study was to determine the effects of pH and PO 4 on the transport of AsO 4 under conditions where reaction and mass transfer rates may be controlling pore-water AsO 4 concentrations. Saturated column transport experiments (pore water velocity = 1 cm h -1 ) were performed using a sand containing free Fe oxides in which a pulse of 73 AsO 4 (133 μM ) was applied in the presence of 0, 13.4, 134, and 1340 μM PO 4 at pH values of 4.5, 6.5, and 85. Breakthrough curves (BTCs) of AsO 4 exhibited low recovery and significant tailing at pH values of 4.5 and 65 in the absence of PO,. Competition between PO 4 and AsO 4 for sorption sites resulted in increased AsO 4 mobility; however, even in the presence of PO 4 levels representing more than 100% of maximum sorption capacity for the columns, significant amounts of AsO 4 remained sorbed to the sand. Application of a continuous PO, pulse to a column containing sorbed AsO 4 resulted in an increase in AsO 4 recovery (>35%); however, even after total PO, loading exceeded the column capacity by more than two orders of magnitude, approximately 40% of the applied AsO 4 remained sorbed to the sand indicating that rates of AsO 4 desorption play an important role in transport of AsO 4 through porous media.

Effect of Colloidal Goethite and Kaolinite on Colorimetric Phosphate Analysis

Filtration through 0.45-µm filters provides a convenient and widely used operational separation between dissolved and particulate P, yet colloidal P is included in the filtrate with true dissolved P. In addition, centrifugation is often used to separate solids from solution during PO4 sorption experiments, but colloids can be present in the supernatant solution. It is not known whether colloid-sorbed PO4 is detected colorimetrically in solutions that have not been pretreated to release colloidal PO4. The objectives of this research were to determine whether colloid-sorbed PO4 is detected colorimetrically and whether the type and size of colloid, the pH during PO4 sorption, or the sorption reaction time affect the extent to which colloid-sorbed PO4 is detected colorimetrically. To accomplish these objectives, 10 mg L−1 suspensions of 40- to 100-nm and 100- to 450-nm kaolinite and goethite were reacted with sodium phosphate solutions (4, 8, and 16 µM PO4) at pH 4.5 and 7 in 10 mM NaCl for 1 and 10 d. The concentration of PO4 that was not detected colorimetrically in the presence of colloids (i.e., nonreactive PO4) was calculated as the difference between PO4 in colloid-free blanks and colloid-containing samples, whereas PO4 sorption was measured in samples that had been ultrafiltered through a 4-nm membrane to separate colloids from solution. Nonreactive PO4, which represents the fraction of sorbed PO4 for which desorption kinetics are slow compared with the colorimetric analysis time, ranged from 2% to >35% of total PO4 (Ptot) and from 20 to 100% of sorbed PO4, depending upon Ptot, colloid type, and reaction time. Nonreactive PO4 was about two times greater after 10-d than 1-d reaction and about 1.4 times greater for large than small colloids, even though small colloids sorbed more PO4. Nonreactive PO4 was greater at pH 4.5 than 7 and was greater for goethite than kaolinite. Nonreactive PO4 was independent of Ptot, although sorbed PO4 increased with increasing Ptot. Thus, each type of colloid apparently has a finite capacity to retain sorbed PO4 in nonreactive sites. Because sorbed PO4 is not uniformly detected colorimetrically, accurate quantitation of the colloid-sorbed PO4 will require additional pretreatmeats such as dissolution of the colloids in strong acid.

Water column and sediment nitrogen and phosphorus distribution patterns in the Florida Keys, USA

Measurements of the distribution patterns of nutrients (ammonium, nitrate, orthophosphate, total N and total P) and chlorophyll concentrations were conducted under an interdisciplinary program known as SEAKEYS, initiated because of concern that anthropogenic nutrients may be impacting Florida coral reefs. Samples were collected along transects that extended from passes or canals to 0.5 km offshore of the outermost reefs. Seven of the transects were either in the Biscayne National Park (BNP) and Key Largo (upper keys) or Seven Mile Bridge/Looe Key (upper part of lower keys) areas, which have the best present-day reef development; the two in the middle keys off Long Key were in an area of minimal reef development where passes allow estuarine Florida Bay water to flow onto the Florida reef platform. Off the upper keys, water column concentrations of N and chl a were elevated near marinas and canals (1 μM NO3, 1 μg/l chl a), but returned to oligotrophic levels (e.g., chl a ⩽ 0.25 μg/l; NO3 ⩽ 0.25 μM; NH4 ⩽ 0.10 μM) within 0.5 km of shore. Phosphorus concentrations, however, were often higher offshore ⩾ 0.2 μM PO4). Sediment interstitial nutrient concentrations decreased from inshore to the offshore reef areas (e.g., ⩾ 100 μM NH4 inshore to ⩽ 50 μM NH4 offshore) and were comparable to those of some presumably pristine coastal and reef carbonate sediments. Sediment bulk N was higher nearshore and decreased steeply offshore ( ⩾ 60 μg-at N/gm sediment to ⩽ 20 μg-at N/gm sediment, respectively); bulk P concentrations (⩽ 6 μg- at P/gm sediment) varied little or exhibited the reverse pattern. Sediment N:P ratios were consistently lower offshore (1–10 vs. 20–40 nearshore). Higher offshore P concentrations are attributed to periodic upwelling along the shelf edge. In the middle keys water column nutrients and chl a concentrations were both higher than those in the upper keys, and there was less of an inshore-offshore decrease than that noted in the upper keys. Sediment nutrients were higher also, and nearshore and offshore areas did not differ. Water column and sediment nutrient concentrations and distribution patterns in the upper part of the lower keys were most similar to those measured in the upper keys. Overall, the present data do not support the contention that reef areas in the upper keys are accumulating elevated loads of land-derived nutrients via surface water flow, but does document moderately elevated nutrient and chl a levels in many developed nearshore areas. Most of the anthropogenic and natural nutrients entering the coastal waters from shore appear to be taken up by near shore algal and seagrass communities before they reach patch reef areas. Further work is needed to determine whether nutrient-enriched ground waters reach the reefs, however these would be expected to cause an enrichment of reef sediments, which was not observed.