Total Algal Biomass Research Articles

The influence of consumer diversity and indirect facilitation on trophic level biomass and stability

The relationship between species diversity and the stability and production of trophic levels continues to receive intense scientific interest. Though facilitation is commonly cited as an essential underlying mechanism, few studies have provided evidence of the impact that indirect facilitation may have on diversity–ecosystem functioning relationships. In this laboratory study, we examined the effect of zooplankton species diversity on trophic structure (total algal and zooplankton biomass) and temporal stability of total zooplankton biomass. We utilized four species of pond zooplankton grown in either monoculture or in polyculture. When comparing responses in polycultures with responses averaged across monocultures, a positive effect of diversity on total zooplankton biomass was observed. This occurred as a result of positive facilitative effects among competing zooplankton. Daphnia pulex, a biomass dominant in monoculture, was negatively affected by the presence of interspecific competitors. In contrast, Diaphanosoma brachyurum, a species that performed poorly in monoculture, was strongly and positively affected by the presence of interspecific competitors, driving positive diversity effects on total zooplankton biomass. Positive temporal covariances among zooplankton were detected in several polyculture replicates, increasing temporal variability of total zooplankton biomass. However, this destabilizing effect was weak relative to effects of high biomass yields in polyculture which caused temporal biomass variability (as measured by the coefficient of variation) to be lower in polyculture relative to monocultures. Zooplankton diversity effects on total algal biomass were not detected. However, increased zooplankton diversity significantly altered the size structure of algae, increasing the relative abundance of large, grazer‐resistant algae.

How protective are refuges? Quantifying algal protection in rock crevices

Summary1. Refuges can be functionally important if they harbour sufficient organisms during disturbance to augment population recovery. I quantified the protection of stream algae in crevice refuges using the applied, severe disturbance of scrubbing. Scrubbing effectively removed visible surface algae, and algae remaining on stones were considered protected.2. In a field experiment, substrata with different quantities of crevice (glass bottles; greywacke; schist; pumice) were incubated in a channelised stream. The possible influence of growth conditions was investigated concurrently; half of the substrata were suspended in the water column, the rest were placed on the bottom (providing differential access to grazers and exposing them to different flow conditions).3. Rougher substrata had greater total algal biomass than smoother substrata; this pattern resulted from more algal biomass in crevices of the rougher substrata. Protection from scrubbing ranged from about 5% of total algal biomass on glass and greywacke to 80% on pumice. In contrast, algal biomass removed by scrubbing was similar among the experimental substrata. Suspended substrata had more chironomid grazers than those on the bottom, and also greater algal biomass, possibly because of high algal concentrations in the chironomid retreats.4. A survey of stones from three rivers supported the experimental results; namely, rough pumice protected more algae from scrubbing than did smoother greywacke.5. In a separate experimental assay, there was no difference in algal growth on agar plates with and without added powdered rock substrata, suggesting that crevice characteristics and not substratum chemistry produced the differences in algal assemblages between rough and smooth surfaces.6. Results indicate that rough stream stones may protect sufficient algae to augment their recovery in streams following disturbance.

Effects of phosphorus loading on interactions of algae and bacteria: reinvestigation of the ‘phytoplankton-bacteria paradox’ in a continuous cultivation system

The effect of different phosphorus loads (LP) on the phosphorus (P) and carbon (C) con- tent (biomass) of algae and bacteria was assessed in continuous culture. We tested if a mixed fresh- water microbial assemblage co-cultured with a phytoflagellate (Cryptomonas phaseolus) would com- ply with the 'phytoplankton-bacteria paradox' (sensu Bratbak & Thingstad 1985). This hypothesis states that the ratio of bacterial to algal abundance changes to the benefit of bacteria with decreasing LP. However, the phenomenon was originally investigated by simultaneously altering LP and micro- bial growth rates, and it is unclear to which extent it can be assigned to either parameter. Therefore, we set up 3 chemostat systems in triplicate at equal dilution rates, but with daily LP of 21, 41 or 62 µg l -1 d -1 (corresponding to 50, 100 and 150 µg P l -1 ). Higher LP led to a 5-fold increase in total algal abundance and biomass but to less than a doubling of these parameters in the bacterial assemblage. Total biomass ratios of bacteria to algae changed from 0.18 to 0.06 with increasing LP, while the bac- teria-algae total phosphorus ratios decreased from 0.80 to 0.17. The cellular C:P ratio of algae remained similar at all P concentrations, whereas the molar C:P ratios of bacterial cells significantly increased at higher LP (from 44 to 73). An enrichment experiment with the 50 µg P l -1 treatment demonstrated that bacteria at the lowest LP were co-limited by P and C, and that increased P stimu- lated mainly the algal fraction. The phytoplankton-bacteria paradox at the level of a mixed microbial assemblage is thus characterised by the following aspects: (1) bacteria profit from their high affinity to P and are better competitors at lower LP; (2) although algae compete with bacteria for P, P-limited algae release extracellular C that stimulates growth of their bacterial competitors; (3) when bacteria depend on algae as their sole source of organic C, this provides a feedback mechanism by which algae limit the abundance of their competitors at higher LP; (4) large oscillations in the bacteria-algae ratios at the lowest LP point to a greater instability of this interaction with stronger P competition. However, bacteria were not able to outcompete C. phaseolus, as algae were their only C source.

Effects of grazer immigration and nutrient enrichment on an open algae‐grazer system

After disturbance, recovery dynamics of local populations depend on arrival rates of immigrants and local growth conditions. We studied the effects of herbivore immigration rates and nutrient enrichment on the dynamics of grazing insect larvae, benthic microalgae, and filamentous macroalgae recovering from low local densities in an open stream system. The two types of algae approximate a trade‐off between capabilities for growing at low resource levels and resisting herbivory. Many microalgae achieve relatively high growth rates at low nutrient levels but are vulnerable to grazers, whereas many macroalgae require high nutrient levels for growth but become increasingly defended with filament growth. We hypothesized that macroalgae should benefit more strongly than microalgae from increasing nutrient levels and decreasing grazer immigration rates, because both conditions increase macroalgal chances to grow into a size refuge from herbivory.We created a gradient of nutrient concentrations and manipulated drift immigration rates of macroinvertebrates. Macro‐ and microalgal biomass and the relative contribution of macroalgae to total algal biomass increased with increasing nutrient enrichment and decreased with increasing grazer immigration. Grazer densities responded positively to nutrient enrichment. The densities of large baetids responded positively to higher immigration rates of large baetids, whereas small baetids and chironomid larvae showed the opposite response. Per capita emigration of small baetids decreased with increasing algal biomass. The data suggest that large baetids negatively affected algal biomass and that small baetid and chironomid densities tracked resource levels set by nutrient enrichment and large baetids. Our experiments highlight the prospects of integrating disturbance with nutrient supply, immigration rates and local trophic interactions (determining recovery trajectories) into conceptual models of open system dynamics. We suggest that recovery trajectories towards micro‐ or macroalgal dominated states may depend on the spatial scale of disturbance relative to the movement ranges of migrating grazers and to nutrient supply.

The Effect of Artificial Destratification on Phytoplankton in a Reservoir

The effects of artificial destratification on limnological conditions and on phytoplankton were surveyed for 6 years (1995-2000) in Lake Dalbang (South Korea), a water supply reservoir receiving nutrients from agricultural non-point sources. In order to reduce odor problems caused by cyanobacterial blooms, six aerators were installed in 1996 and operated regularly during the warm season. Aeration destratified the water column of the reservoir and produced homogeneous physical and chemical parameters. The maximum surface temperature in summer decreased from 28.9 °C before aeration to 20.0-26.4 °C after aeration, whereas the maximum hypolimnetic temperature increased from 8.0 to 17.0-23.7 °C. Despite these changes, surface water concentrations of total phosphorus (TP) and chlorophyll a(CHLA) and their seasonal patterns did not change with destratification. Phosphorus loading was concentrated in heavy rain events during the summer monsoon, and TP and CHLA reached maximal concentrations in late summer after the monsoon. Because the hypolimnion was never anoxic prior to aeration, internal loading did not seem to be substantial. Cyanobacteria were the dominant phytoplankton in summer before aeration, but diatoms replaced them after operation of the aerator. Cyanobacteria blooms were eliminated. In contrast, total algal biomass in the water column (as CHLA integrated over depth) increased from 190 mg m−2 in 1995 to 1150, 300, 170, and 355 mg m−2 in 1997, 1998, 1999, and 2000, respectively. The increased ratio of mixing depth to euphotic depth to 2.5 may have resulted in a net reduction in the amount of underwater irradiance experienced by phytoplankton cells, and this may have favored the switch to diatom dominance. Furthermore, the mixing may have allowed diatoms to flourish in summer by lowering their settling loss that would be critical in stratified water columns. In conclusion, the destratification in this reservoir was effective in preventing cyanobacteria blooms, but not in reducing the total algal standing crop.

Picoplankton community structure along the northern Iberian continental margin in late winter-early spring

The surface distribution of autotrophic and heterotrophic picoplankton was assessed in 24 transects perpendicular to the coast along the N and NW Iberian peninsula shelf in late winter and early spring 2002. Community structure was analyzed by flow cytometry (FC) and found to be strongly influenced by hydrography. Typical late winter conditions were found during the survey, characterized by the presence of the poleward Portugal coastal counter current (PCCC) in the west and an increasing stratification eastwards. Cyanobacteria (mostly Synechococcus) dominated at low chlorophyll a (Chl a) concentration whereas both the total and relative abundance of picoeukaryotes generally increased with total phytoplankton biomass. Differences in the cell size of most FC-defined picoplanktonic groups were also observed along the longitudinal and coastal‐offshore gradients. The presence of Prochlorococcus (<10 3 cells mL � 1 ) coincided with the core of the PCCC and its significant correlation with salinity suggests its possible use as a tracer of this current. Two groups of heterotrophic bacteria were distinguished according to their relative DNA content. High DNA bacteria dominated the community (60¯� 1% SE of total numbers), reaching maximum values in areas under riverine influence with presumed higher inputs of organic matter. Picoplankton biomass was dominated by heterotrophic bacteria in the western region (58¯� 3%) while autotrophic groups contributed on average 66¯� 2% in the southern Bay of Biscay. The heterotrophic bacteria to phytoplankton biomass ratio decreased significantly along the measured range. Yet showing regional differences, the estimated contribution of picophytoplankton to total algal biomass was high (mean 59¯� 4%), indicating the important role of small cells at the onset of the spring bloom in these temperate shelf waters.

Copepods act as a switch between alternative trophic cascades in marine pelagic food webs

AbstractA recent meta‐analysis indicates that trophic cascades (indirect effects of predators on plants via herbivores) are weak in marine plankton in striking contrast to freshwater plankton (Shurin et al. 2002, Ecol. Lett., 5, 785–791). Here we show that in a marine plankton community consisting of jellyfish, calanoid copepods and algae, jellyfish predation consistently reduced copepods but produced two distinct, opposite responses of algal biomass. Calanoid copepods act as a switch between alternative trophic cascades along food chains of different length and with counteracting effects on algal biomass. Copepods reduced large algae but simultaneously promoted small algae by feeding on ciliates. The net effect of jellyfish on total algal biomass was positive when large algae were initially abundant in the phytoplankton, negative when small algae were dominant, but zero when experiments were analysed in combination. In contrast to marine systems, major pathways of energy flow in Daphnia‐dominated freshwater systems are of similar chain length. Thus, differences in the length of alternative, parallel food chains may explain the apparent discrepancy in trophic cascade strength between freshwater and marine planktonic systems.

Plankton community responses in earthen channel catfish nursery ponds under various fertilization regimes

We evaluated fertilizer effects on phytoplankton and zooplankton concentrations in channel catfish Ictalurus punctatus nursery ponds. In 2001, three ponds were not fertilized and three ponds were fertilized with inorganic (8.4 kg/ha N, 2.0 kg/ha P followed by twice a week applications at half the initial rate) and organic fertilizer (224 kg/ha cottonseed meal followed by once a week applications of 28 kg/ha) for 4 weeks. Total P and soluble reactive P (SRP) were significantly higher in fertilized ponds ( P<0.05), however no differences in algal populations resulted. Of the zooplankton, only copepod nauplii were higher in fertilized ponds. In 2002, 26 ponds were used. Sixteen ponds were newly constructed; 10 ponds were old (>10 years). Within each age group, treatments of no fertilizer (controls), organic fertilizer only, inorganic fertilizer only, or both organic and inorganic fertilizers were used for 3 weeks. In 2002, the same P but higher N rates were used (20.2 kg/ha N followed by twice a week at half rate). Several differences occurred among treatments ( P<0.05). Both total P and total N were significantly higher in ponds fertilized with inorganic fertilizer and both inorganic and organic fertilizers than in control ponds and organically fertilized ponds. Also, total P, total N, SRP, and nitrite were all significantly higher in old ponds than in new ponds. All phytopigment concentrations were also higher in old ponds than in new ponds. Chlorophyll a (total algal biomass), lutein (green algal biomass), and fucoxanthin (diatom biomass) were significantly higher in ponds fertilized with inorganic fertilizer or both inorganic and organic fertilizers than in control or organically fertilized ponds. Old ponds contained significantly more rotifers, copepod adults, and ostracods than new ponds. Rotifer and ostracod concentrations were not different among fertilizer treatments. Copepods adults and nauplii were significantly increased in ponds fertilized with both organic and inorganic fertilizers than in organically fertilized ponds. Cladocerans were significantly higher in inorganically fertilized ponds than in all others. Results indicated little benefit of organic fertilizer addition at the rates used. Also, ponds appear to be more N limited than P limited. Old ponds are more fertile than new ponds. Applying only inorganic fertilizer at an initial rate of ∼20 kg/ha N and 2 kg/ha P, followed by subsequent applications of half the initial rate for 3–4 weeks increased zooplankton concentrations desirable for fry with larger mouth gapes.

Lake Maggiore (N. Italy) trophic history: fossil diatom, plant pigments, and chironomids, and comparison with long-term limnological data

The availability of long-term series of chemical and biological data and the eutrophication/oligotrophication history of Lake Maggiore allows an attempt to correlate the registered changes with sedimentary records in several sediment cores. Documentary and palaeolimnological data were used to calibrate two important suites of sedimentary indicators of phytoplankton, diatoms and algal pigments. Diatom assemblages in the sediment cores precisely reflect the pelagic diatom development for approximately the last century. Prediction of total algal biomass from the profile of the ubiquitous β-carotene and some algal groups in certain period of lake development (e.g. diatoms, Cryptophyta, cyanobacteria) was good, whereas the comparison of taxa-specific carotenoids and algal biomass standing stocks (as cell biovolume) in some case revealed poor correspondence. Selective carotenoid losses, taxa production, and mechanisms controlling pigment sedimentation are factors that biased the comparison. However, pigment concentrations and algal biovolumes are different units but equally valid. The use of fossil pigments complements other studies and provides more detailed information on algal development. A sub-fossil chironomid profile agrees well with the general trophic reconstruction as inferred from the pigment and diatom data, adding more details on changes in littoral substratum, water-level fluctuation and flood events. Models to infer primary productivity and total phosphorus concentration in lake water from sedimentary pigments and diatom assemblages are tested: in the case of the TP reconstruction, reliable results were obtained in this case for the period of high trophic state and for the last decade. During the recovery phase of the 1980s, unexpected high abundance of Stephanodiscus minutulus leads to strong overestimation of TP concentrations. Similarly, the reconstructed primary productivity only disagrees with the experimental data for some years in the last decade. Sensitivity of the sedimentary pigment model as well as the relatively reduced sampling dates likely explains this discrepancy. Similarities are also evident in the temporal diatom assemblage variations of an additional three sub-alpine Italian lakes. As well, the palaeolimnological reconstruction for Lake Maggiore parallels that for Lake Constance, another large sub-alphine lake located north of the Alps.

Modelling the effects of artificial mixing and copper sulphate dosing on phytoplankton in an Australian reservoir

AbstractAn artificially destratified reservoir was simulated with the freshwater phytoplankton model PROTECH (Phytoplankton Responses To Environmental Change). The chosen site for validation was a highly managed drinking water supply reservoir (Myponga Reservoir, South Australia). Chemical dosing using copper sulphate (CuSO4) and artificial mixing via an aerator and two raft‐mounted mechanical surface mixers (hereafter referred to as surface mixers) are used at Myponga to manage water quality, in particular the threat of cyanobacteria growth. The phytoplankton community was adequately modelled and showed that the community was dominated by species tolerant of low light doses (R‐type strategists). The light limitation in the water body was found to be the controlling factor on phytoplankton succession. Subsequently, small fast‐growing species and larger motile phytoplankton (C and CS‐type, respectively) do not have the opportunity to dominate under all simulated conditions, diminishing the need for CuSO4 dosing. These simulations demonstrated that the individual and combined impact of the management strategies reduces the total algal biomass, but have minimal effect upon phytoplankton functional‐type succession, and R‐type species continued to dominate under all simulated scenarios. It was concluded that, due to the light‐limitation and current nutrient availability in Myponga Reservoir, the probability of persistent populations of undesirable scum‐forming cyanobacteria is minimal, even in the absence of artificial control.

Why plankton communities have no equilibrium: solutions to the paradox

In a classical paper, Hutchinson (1961) argued that the large number of species in most plankton communities is remarkable in view of the competitive exclusion principle, which suggests that in homogeneous, well-mixed environments species that compete for the same resources cannot coexist. Few ideas in aquatic ecology have evoked more research than this `paradox of the plankton'. This review is an effort to put the main solutions to the paradox that have been proposed over the years into perspective. Hutchinson himself already suggested that the explanation could be that plankton communities are not in equilibrium at all due to weather-driven fluctuations. Subsequent research confirmed that such externally imposed variability can allow many species to coexist. Another important point is that in practice the homogeneous well-mixed conditions assumed in the competitive exclusion principle hardly exist. Even the open ocean, for instance, has a spatial complexity resulting from meso-scale vortices and fronts that can facilitate coexistence of species. Perhaps most excitingly, theoretical work on species interactions has given a counter-intuitive new dimension to the understanding of diversity. Various competition and predation models suggest that even in homogeneous and constant environments plankton will never settle to equilibrium. Instead, interactions between multiple species may give rise to oscillations and chaos, with a continuous wax and wane of species within the community. Long-term laboratory experiments support this view. This chaotic behavior implies among other things that plankton dynamics are intrinsically unpredictable in the long run when viewed in detail. Nonetheless, on a higher aggregation level, indicators such as total algal biomass may show quite regular patterns.

Phytoplankton assemblage structure and primary productivity along 170°W in the South Pacific Ocean

Phytoplankton pigments were measured using HPLC during non-ENSO conditions in mid-summer along a South Pacific transect from 67° S to the equator along 170° W. Highest concen- trations of chlorophyll a (chl a) occurred in the Polar and the Subtropical Fronts (PF and STF, respec- tively) with concentrations exceeding 500 ng l -1 . In the STF, there was a distinct subsurface chl a maximum (SCM) at 40 m, which gradually deepened northward to 120 m in the Subtropical Conver- gence Zone. Northwards, the SCM shoaled to about 30 m in the Equatorial Zone (EZ). Relatively high concentrations of fucoxanthin and 19'-butanoyloxyfucoxanthin occurred in the nutrient-rich waters south of the Subantarctic Front, and CHEMTAX analyses indicated that diatoms, chrysophytes, pelagophytes, and haptophytes dominated the phytoplankton assemblage. Northward of the PF to the STF, where silicate concentrations were <1 µM, pelagophytes and coccolithophorids dominated the water column; diatoms were virtually absent, and Phaeocystis, prasinophytes, cryptophytes, and chlorophytes contributed significantly to the total algal biomass. Phaeocystis populations were dom- inant at or below the 1% light level. In the South Pacific Gyre (SPG), Synechococcus (Syn) and Prochlorococcus (Pro) were major components of the phytoplankton assemblage with Pro dominant as indicated by both flow cytometry and by the ratio of divinyl chl a:total chl a (0.43 ± 0.07). Photo- acclimation by Pro in the SPG was pronounced, with a higher average divinyl chl a per cell ratio in the SCM (1.1) relative to values (0.1) in the upper waters (0 to 100 m). Primary production rates exceeding 1 µg C l -1 h -1 occurred at the surface in the STF. Surface primary production rates were generally < 0.4 µg C l -1 h -1 across the SPG, but exceeded 1.4 µg C l -1 h -1 at the equator. In the EZ, Pro dominated the phytoplankton assemblage, but Phaeocystis and prasinophytes were also major constituents of the assemblage.

Sea ice biological communities and nutrient dynamics in the Canada Basin of the Arctic Ocean

Salinity, concentrations of silicate (Si), phosphate (P), and chlorophyll a (chl a), and the species composition of floral and faunal communities were assessed in multi-year (MY) and first-year (FY) ice, and at the water–ice interface, during the SHEBA ice camp drift in the Canadian Basin of the Arctic Ocean, October 1997–October 1998. Mean integrated salinity values varied from 0.1±0.8‰ within the snow–ice boundary to 3.2±1.46‰ at the water–ice interface in the MY ice. Salinity of FY ice increased with increasing ice thickness, from 0.41±0.19‰ in October to 3.39±1.75‰ in March. We found very low concentrations of both Si and P in MY and FY ice in late fall (<0.01 μM), after the summer season of 1997. Mean integrated concentration of nutrients increased with the freeze-up of ice, reaching maximum values in the summer of 1998. Ice algal biomass was dominated by diatoms in the MY and FY ice, with more pennate than centric species. Species of dinoflagellates and green algae were subdominant. Green algae, typically confined to the upper part of the ice, were distributed throughout the ice thickness. During summer, the brackish-water green alga Ulothrix implexa formed long rope-like tufts attached to the hull of the icebreaker; development of this species has not previously been observed in the central Arctic Ocean. Biomass, cell abundance and chl a concentrations were high in the bottom sections of both MY and FY ice during the pre-freezing period, then decreased in winter and reached maximum values in spring and summer. The total algal biomass showed a pattern similar to chl a dynamics, decreasing from 1.7 μg C l −1 in October to values <0.1 μg C l −1 in winter. Algal carbon biomass and chl a concentration co-varied significantly in the FY ice ( r 2=0.79), but less so in the MY ice ( r 2=0.49). An unexpected feature with respect to sea ice invertebrates was the absolute absence of living interstitial fauna within the interior of MY and FY sea ice. In addition, the under-ice fauna was species-poor. Comparisons of SHEBA results with historical data showed that the physical–chemical characteristics of sea ice and the biological structure of ice communities found in this study were very different from conditions during the 1970s. It is likely that the changes resulted from increased melting of the arctic ice pack over the last two decades.

Stability of diatom composition in a variable lake environment: Lake Chascom�s, Argentina

Water and surficial sediment samples of Lake Chascomus and its tributaries were analyzed in order to relate changes in diatom community structure to chemical variables. Over the course of 13 months of sampling, the lake exhibited major changes in water level (1.15–1.98 m average depth), total dissolved solids (821–1972 mg l−1), silica (0.098–8.22 mg l−1), and total algal biomass (21.4–145.9 μg Chl a l−1). However, despite these large fluctuations, the diatom species composition was relatively stable. The dominant species in the water column was always Synedra berolinensis (68.9%–90.1% total frustules), with Fragilaria construens and F. brevistriata as subdominants. In the sediments the latter two species dominated the frustule counts. These results indicate an unusual floristic stability of this eutrophic ecosystem, with persistent dominance by broadly tolerant, generalist species.

Response of phytoplankton communities to liquid creosote in freshwater microcosms

AbstractWe assessed the response of phytoplankton communities in aquatic microcosms to single applications of liquid creosote. The creosote was applied to 14 microcosms at concentrations ranging from 0.06 to 109 mg/L. Two microcosms served as controls. Phytoplankton samples were collected from each microcosm one week and 1 d before treatment and at 7 and 21 d after treatment. Temporal changes (response–recovery) in phytoplankton community composition were assessed with principal response curves. Creosote had no direct toxic effect on the phytoplankton community based on total abundance and number of taxa. Population levels declined in all treatments between day −1 and day 7, but this trend mirrored a similar decline in the control microcosms. At both 7 and 21 d after treatment, population densities and number of taxa in most treatments exceeded those in the controls and exhibited a parabolic relationship relative to creosote concentration. This relationship was most pronounced at 21 d, at which time population densities and number of taxa at intermediate concentrations were up to twice those at low and high concentrations. This response pattern seems to represent an indirect response to impacts on zooplankton and a corresponding reduction in grazing pressure. In contrast, total algal biomass declined 52 to 97% relative to the controls at all but the lowest creosote concentration at 7 d. This apparent decline was due to a significant proliferation of the alga Closterium moniliforme in the controls and low creosote concentration. At 21 d, no difference was found in total biomass between treated and control microcosms. The results of this study suggest that creosote does not pose a significant direct risk to phytoplankton at concentrations likely to be encountered in most contaminated aquatic environments; however, stimulation of algal populations could occur in situations of long‐term chronic exposure or spill events that remove predatory zooplankton populations.

THE EFFECTS OF PREY HETEROGENEITY AND CONSUMER IDENTITY ON THE LIMITATION OF TROPHIC-LEVEL BIOMASS

The effects of prey heterogeneity and consumer identity on the strength of predator limitation of prey biomass were explored experimentally under controlled laboratory conditions. In this study, I utilized a model aquatic community composed of zooplankton as top predators, algae as prey, and nutrients as basal resources. To examine the effects of prey heterogeneity, I created a food chain initially composed of a single edible prey and a food web initially composed of a diverse assemblage of algae. These two prey treatments were then fully crossed with two predator treatments (a large-bodied zooplankter, Daphnia pulex, and a small-bodied species, Ceriodaphnia quadrangula), and two levels of productivity. Prey heterogeneity had clear effects on the ability of predators to limit overall prey biomass. In food chains, predators had strong negative effects on algae, and algal biomass exhibited a narrow response to enrichment. In contrast, predator limitation was weak in food webs with the consequence that predator and prey biomass both showed positive increases with productivity. The prey community in food webs also exhibited a striking increase in the relative abundance of large inedible algae with enrichment, in keeping with model predictions. These results indicate that prey heterogeneity can have substantial effects on predator–prey dynamics and trophic structure and can serve to shift systems from strong top-down control to ones in which prey are colimited by predators and resources. Comparisons between top predators showed that Daphnia, compared to Ceriodaphnia, more strongly limited the biomass of large algae in food webs at high productivity and total algal biomass in all nutrient-enriched treatments (both chains and webs). Thus, consumer identity and ecological context (productivity and heterogeneity of prey communities) may mediate the strength of zooplankton–algae interactions and the efficacy of trophic cascades.

SOME BIOTIC AND ABIOTIC VARIABLES CONTROLLING PRIMARY PRODUCTIVITY IN HYPERTROPHIC LAKE (LAKE QARUN-EGYPT)

Primary productivity was measured monthly in Lake Qarun during 1993 using C technique and the annual average value was 105.5 mg Cm h . The study revealed that both biotic and abiotic variables have affected the primary productivity. Phytoplankton biomass, as biotic factor, was represented by total Chlorophyll a (annual average of 26 mg m 3). Chlorophyll a was fractionated into two major groups based on their availability to herbivorous zooplankton; nanoplankton ( 20fi) or inedible algae. Nanoplankton was dominated and represented by about 69,4% of total phytoplankton, while netplankton was about 30.6%. Size fraction ratio was controlled by their relative size-specific growth rates not by herbivorous zooplankton. There was a linear relationship between the relative contribution of nanoplankton and total algal biomass (r=0.96, p>95). Primary productivity has been affected by nanoplankton more than any other biotic variable. Abiotic parameters (physical and chemical) were also studied and discussed in relation to its effect on primary productivity. The statistical analysis cleared that water temperature and NO3-N were the main factors regulating primary productivity among all ecological parameters tested.

An experimental analysis of the effects of herbivory and nutrient enrichment on benthic community dynamics on a Hawaiian reef

Reduced herbivory and enhanced nutrient concentrations have both been suggested as probable mechanisms driving phase shifts from coral to algal dominance on tropical reefs. While there is considerable information regarding the effects of herbivores on reefs, there is little experimental evidence of nutrient effects or the interactive effects of both of these factors. This study experimentally tested the role of these factors on benthic community structure on a coral-dominated reef in Hawaii. A randomized factorial block design was used to simultaneously investigate the effects of nutrient enrichment and herbivore exclusion on the development of benthic algal communities on artificial surfaces over a 6-month time period. Total algal biomass was greatest on settlement surfaces exposed to both nutrient enrichment and herbivore exclusion simultaneously. Fleshy algal biomass was greatest on surfaces removed from grazing whereas calcareous biomass was greatest on surfaces exposed to nutrient enrichment. Control surfaces exhibited consistently less total, fleshy and calcareous algal biomass than that on any of the experimental surfaces. Microinvertebrates were most abundant on surfaces within herbivore exclusion treatments but increased in number on all settlement surfaces over time. Sediment accumulation was positively correlated with fleshy algal biomass and was most abundant on surfaces within herbivore exclusion treatments; there was no pattern in sediment accumulation over time. This research demonstrates that on short time scales (less than 6 months), nutrient enrichment and herbivore exclusion can independently and interactively support shifts in benthic algal community structure on a Hawaiian reef.

Influence of the density of Caulerpa prolifera (Chlorophyta) on the composition of the macrofauna in a meadow in Algeciras Bay (Southern Spain)

The coverage and biomass fluctuations of Caulerpa prolifera (Chlorophyta) were used to assess the influence of the alga on the composition of its associated animal community, on a small spatial scale. Bymeans of ordination analysis, based on species abundance, a series of groups, related to the different algal density recorded, were obtained: those stations with highest algal biomass (stations 1, 2 and 8) and the remaining stations (medium and low biomass). According to a canonical correspondence analysis, total algal biomass, coverage and percentage of organic matter explained the differences in the composition of the macrobenthic communities. The response of some species to the spatial distribution of the alga are also discussed. In conclusion, the patchiness of Caulerpa beds cause a high degree of heterogeneity in a spatially reduced system, suggesting the importance of these sites for the biodiversity in coastal ecosystems.

Response of zooplankton communities to liquid creosote in freshwater microcosms

We assessed the response of phytoplankton communities in aquatic microcosms to single applications of liquid creosote. The creosote was applied to 14 microcosms at concentrations ranging from 0.06 to 109 mg/L. Two microcosms served as controls. Phytoplankton samples were collected from each microcosm one week and 1 d before treatment and at 7 and 21 d after treatment. Temporal changes (response-recovery) in phytoplankton community composition were assessed with principal response curves. Creosote had no direct toxic effect on the phytoplankton community based on total abundance and number of taxa. Population levels declined in all treatments between day -1 and day 7, but this trend mirrored a similar decline in the control microcosms. At both 7 and 21 d after treatment, population densities and number of taxa in most treatments exceeded those in the controls and exhibited a parabolic relationship relative to creosote concentration. This relationship was most pronounced at 21 d, at which time population densities and number of taxa at intermediate concentrations were up to twice those at low and high concentrations. This response pattern seems to represent an indirect response to impacts on zooplankton and a corresponding reduction in grazing pressure. In contrast, total algal biomass declined 52 to 97% relative to the controls at all but the lowest creosote concentration at 7 d. This apparent decline was due to a significant proliferation of the alga Closterium moniliforme in the controls and low creosote concentration. At 21 d, no difference was found in total biomass between treated and control microcosms. The results of this study suggest that creosote does not pose a significant direct risk to phytoplankton at concentrations likely to be encountered in most contaminated aquatic environments; however, stimulation of algal populations could occur in situations of long-term chronic exposure or spill events that remove predatory zooplankton populations.