Increase In Water Transparency Research Articles

The zooplankton community of Lake Abo Zaabal, a newly-formed mining lake in Cairo, Egypt

Abo Zaabal Lake, a new mining lake at the outskirts of Cairo, receives domestic sewage from a nearby village. We examined its chemical and biological properties in order to develop a plan to improve its ecosystem services. The pH ranged between 7.8 and 8.5, salinity was 17 ppt and conductivity was 28 mS cm−1, indicating brackish water. The pelagic zooplankton community comprised Rotifera (16%), Protozoa (Ciliophora and Rhizopoda) (18%) and Copepoda (16%). Cladocera were seldom recorded. Hexarthra, Brachionus and Rotaria were the dominant rotifer taxa. Several characteristics — including the community composition, the dominance of small ciliates and nauplii, the abundance of Cyanobacteria, and the absence of macrophytes — indicated that it is a severely eutrophic lake. To improve the water quality we recommend that: (1) domestic sewage inflow should be stopped so as to reduce nutrient loading into the basin; (2) the growth of rooted macrophytes should be encouraged so as to increase water transparency by reducing phytoplankton biomass through competition for nutrients; and (3) piscivorous fish should be introduced to control zooplanktivorous fish in the lake and thus promote the growth of large filter-feeding zooplankton.

Macrophyte–phytoplankton interactions: The relative importance of allelopathy versus other factors

Submerged aquatic macrophytes are important in shallow aquatic ecosystems because they stabilize the macrophyte-dominated state by increasing water transparency in various ways. One of these is the excretion of allelopathic substances inhibitory to phytoplankton, but it is still controversial whether this mechanism can be important in the field. We developed a model that describes phytoplankton growth including the effects of two different macrophytes ( Chara sp. and Stratiotes aloides), which are both known to excrete allelopathic substances. With this model we analysed singular and combined effects of shading (1), sediment resuspension (2), competition for nutrients (3) and allelopathy (4) on the development of phytoplankton. Both macrophyte species had a considerable effect on phosphorus levels in the water. Shading of phytoplankton by Chara sp. was estimated to be negligible. This macrophyte species has a growth form close to the sediment. The allelopathic activity of Chara sp. caused a phytoplankton growth reduction of ca. 5–10%, while the most pronounced effect seems to be related to reduction of resuspension. S. aloides floats on the water surface during spring and summer. Therefore, it has a potentially high shading effect. Additionally, this species has a relatively strong allelopathic capacity, sometimes resulting in a reduction of phytoplankton between 50 and 80%. The model suggests that the relative contribution of allelopathy in situ is low or negligible for charophytes. Only for macrophyte species that have a high allelopathic potential, like S. aloides, it may be a significant effect.

Management Options and Effects on a Marine Ecosystem: Assessing the Future of the Baltic

We are using the coupled models in a decision support system, Nest, to evaluate the response of the marine ecosystem to changes in external loads through various management options. The models address all the seven major marine basins and the entire drainage basin of the Baltic Sea. A series of future scenarios have been developed, in close collaboration with the Helsinki Commission, to see the possible effects of improved wastewater treatment and manure handling, phosphorus-free detergents, and less intensive land use and live stocks. Improved wastewater treatment and the use of phosphorus-free detergents in the entire region would drastically decrease phosphorus loads and improve the marine environment, particularly the occurrence of cyanobacterial blooms. However, the Baltic Sea will remain eutrophic, and to reduce other effects, a substantial reduction of nitrogen emissions must be implemented. This can only be obtained in these scenarios by drastically changing land use. In a final scenario, we have turned 50% of all agricultural lands into grasslands, together with efficient wastewater treatments and a ban of phosphorus in detergents. This scenario will substantially reduce primary production and the extension of hypoxic bottoms, increase water transparency in the most eutrophied basins, and virtually eliminate extensive cyanobacterial blooms.

Peat erosion and atmospheric deposition impacts on an oligotrophic lake in eastern Ireland

The Holocene diatom and pollen records from Kelly’s Lough have been analysed to determine the timing and extent of the acidification in this upland lake. The pollen data during the early Holocene reflect the typical vegetation changes that occur in sediments throughout Ireland during this period. The diatom record begins by being dominated by circumneutral and acidophilous benthic forms. Later tychoplanktonic Aulacoseira species begin to expand and dominate indicating increased water transparency following the stabilization of catchment soils. Peatland development in the catchment is evident from approximately 6,450 cal year BP. The main change in the diatom assemblages at this time is the decline of Aulacoseira species and expansion of periphytic species. At around 1,450 cal year BP, loss-on-ignition (LOI) values, Calluna pollen and microscopic charcoal all increase suggesting the initiation of a major phase of peat erosion and an increased inwash of organic matter to the lake. Lake acidity changed significantly although the initial acidification is very subtle as indicated by the diatom-inferred pH record. Changes in the diatom assemblages might be largely the result of increasing erosion and inwash of organic matter from the catchment to the lake leading to reduced water transparency and more acidic conditions. The diatom flora remains relatively stable until the mid-twentieth century when more acidibiontic species increase. These diatom changes result in the reconstructed pH curve showing a moderate recent acidification from pH 5.7 to 5.1. About half of the total change in pH took place by around the late 1960s. The lowest diatom-inferred pH value occurs in the late 1970s, and parallels the peak in SO2 emissions in Ireland. Acidic conditions seem to have prevailed in Kelly’s Lough throughout its entire history and alkalinity has been low or absent for much of the time. However, soil acidification and inwash of organic acids from peatlands are not a sufficiently effective mechanism to explain the low pH levels found today in Kelly’s Lough. The effect of acid deposition on the waters of Kelly’s Lough is clear and it has probably caused these already naturally acid waters to acidify further.

Effects of Changing Rainfall on the Limnology of a Mediterranean, Flowthrough‐Seepage Chain of Lakes

AbstractRelationships between groundwater and lake ecology are often overlooked, but they may be strong, particularly in seepage lakes. As a result, the nature and degree of groundwater effects on lakes are usually neglected. In this study interactions among rainfall, groundwater and surface water and their limnological effects were traced seasonally for two years of changing rainfall in a Spanish flowthrough, seepage lake complex. Cumulative rainfall dictated recharge of groundwater with delays of nine months. Groundwater discharge, in turn, increased surface discharge downstream. Mediated by the geographical setting of lakes, both fluxes impinged on lake water renewal time, but effects of the latter on limnological variables were much stronger at the district scale than at the single lake scale. These water‐renewal effects included the following: decreasing salinity, total phosphorus concentration and phytoplankton biomass and increasing water transparency and total nitrogen concentration as water renewal shortened, the nitrogen effect arising because of nitrate‐rich water entering the lakes as groundwater levels rose. This complex response of a Mediterranean lake district to water availability may also be expected in cold temperate lakes as climate change effects become stronger. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Appearance and evolution of marine benthic communities in the Early Palaeozoic

The development of marine benthic communities in the Early Palaeozoic occurred mainly in the shallow water epicontinental seas. It included those stages of the Cambrian and Ordovician evolutionary radiations that were dominated by a high rate of morphogenesis, when new food and territory resources were not limited. This provided many opportunities for coadaptation of emerging organisms. At the time of the Cambrian radiation, the body plans of all animals were formed, while in the Ordovician, the maximum rank of emerging taxa did not exceed the level of class. The beginning of each radiation was explosive. Vendian benthic communities developed in cold seas and in the shallowest areas of warm seas, where organic matter from the surface layers was available at the bottom because of the absence of a thermocline. The Cambrian radiation began with the appearance of pelagic suspension feeders, because of which much of the primary production could penetrate the thermocline and settle at the bottom. This allowed the occupation of warmer seas and greater depths. At the same time, the productivity of the pelagic region sharply increased because of the emergence of positive feedback between the producers and consumers in the water, leading to increased water transparency and elongation of trophic chains. Arthropods, the first suspension feeders, were the launch group of the Cambrian radiation. Cambrian benthic suspension feeders could seize only the smallest particles, mostly bacteria, and dissolved organic matter. This food resource was contained in the thin bottom water layer. Therefore, the food grasping structures of all the Cambrian suspension feeders were near the bottom, without forming tiers. The Ordovician evolutionary radiation began with the launch of the Pelmatozoan echinoderms, which were the first benthic suspension feeders to begin feeding on plankton. The exploitation of this resource led to the creation of a 1-m tier above the bottom and an increase in their calcite productivity. Positive feedback emerged between the grounds and the community of its inhabitants and considerably changed the composition and diversity of grounds, which sharply increased the diversity of benthos. The appearance of positive feedback between different components of ecosystems resulted in explosive evolution in both the Cambrian and Ordovician.

Optical changes associated with cyanobacterial bloom termination by viral lysis

Optical changes that accompanied a collapse of the population of filamentous cyanobacteria from a shallow, eutrophic lake were studied in laboratory-scale enclosures (LSEs). The experimental conditions are known, from previous work on these systems, to cause a dramatic collapse of the dominant algal or cyanobacterial species, which in turn can be associated with viral activity. Within 2 weeks of continuous addition of nutrient-rich growth medium, near-complete collapse of the dominant population occurred over the span of a few days. The collapse was repeatedly and reproducibly observed and was primarily characterized by a marked increase in water transparency. Scattering of light decreased by ∼80%, absorption decreased by 20–80%. There was high similarity in optical changes between several experiments, carried out in different seasons. An increase of dissolved material and submicron-sized particles (SMP) that showed chlorophyll a (Chl a) absorption was observed during the collapse. The phycocyanin (PC): Chl a ratio and phaeopigment : Chl a ratio proved to be good indicators of the observed collapse. Reflectance spectra that were modelled using a constant volume-scattering function indicated that mass mortality of this magnitude can be detected in natural systems using current remote sensors.

The Role of Filtering Zooplankton in De-Eutrophication of Water Bodies

The filtration capacity of large-size cladoceras is examined. Cladoceras Simocephalus vetulus with a biomass of 1.6 g/l in flow-through ecosystems isolated from ichthyifauna can reduce phytoplankton biomass from 2.89 to 0.12 mg/l within 1 h. Particularly, the biomass of blue-green algae Anabaena spiroides decreases from 0.14 to 0.01 mg/l, and their filaments shorten. In the period when phytoplankton biomass in the water body was at maximum, the activity of cladoceras (biomass of 2.1 g/l) resulted in a decrease in this biomass in flow-through ecosystems by a factor of 13.6 (down to 0.34 mg/l). The decrease in phytoplankton biomass in flow-through ecosystems facilitated changes in its composition in the water body (an increase in the share of green algae) and was accompanied by an increase in water transparency.

The Wall of Green: The Status of Cladophora glomerata on the Northern Shores of Lake Erie's Eastern Basin, 1995–2002

The biomass, areal coverage, algal bed characteristics, and tissue phosphorus concentrations of Cladophora glomerata were measured at 24 near shore rocky sites along the northern shoreline of Lake Erie's eastern basin between 1995–2002. Midsummer areal coverage at shallow depths (≤ 5 m) ranged from 4–100%, with a median value of 96%. Cladophora biomass began accumulating at most sites during early May, and achieved maximum values by mid-July. Peak seasonal biomass ranged from < 1 to 940 g/m 2 dry mass (DM), with a median value of 171 g/m 2 DM. Nearshore water concentrations of total phosphorus (TP) were lower than during pre-phosphorus abatement years. However, Cladophora biomass levels were similar to reported values in those years. The midsummer “die off” occurred shortly after the biomass peak, when water temperatures neared 22.5°C. Areal coverage declined after die-off to < 10%, mean filament lengths declined from 33 cm to < 1 cm, and mean biomass declined to < 1 g DM/m 2. Tissue phosphorus varied seasonally, with initial high values in early May (0.15 to 0.27% DM; median 0.23% DM) to midsummer seasonal low values during peak biomass (0.03 to 0.23% DM; median 0.06% DM). Cladophora biomass is sensitive to changes in phosphorus and light availability, and reductions in biomass previously achieved through phosphorus control may now be reversed because of increased water transparency and phosphorus availability to the benthos following establishment of dreissenids.

Periphyton: a primary source of widespread and severe taste and odour

In the last decade, a late summer-fall taste and odour problem has been a prolonged and annual event in the St Lawrence River (SLR). Earlier work identified the earthy/musty compounds geosmin and particularly, 2-methylisoborneol (GM-MIB), and ruled out Lake Ontario as a major source, but did not identify the biological origins. In 2000, we investigated the source(s) and underlying causes. We sampled littoral sites in the SLR near Cornwall, ON, and found that macrophytes (or associated biofilms) may be primary GM sources. Zebra mussel homogenate yielded low GM-MIB levels, but several associated actinomycetes generated high in vitro amounts. Periphyton from rocks showed significant yields, with cell-bound GM-MIB up to one hundred times the levels in overlying water. In 2001, we followed seasonal changes at some of these sites. Periphyton GM-MIB showed intriguing spatial and temporal patterns. Several cyanobacteria in these biofilms were identified as potential odour sources, notably Oscillatoriales. We conclude: i) periphyton is a major odour source in the SLR; ii) other biota such as macrophytes and mussels may also contribute; iii) seasonality in GM-MIB production and ratios indicate changes in cell production and/or taxa in response to environment. These results may account for the recent onset of the problematic odour events, which represent chemical signals of the increased water transparency and littoral surface area following the widespread dreissenid mussel invasion to the Great Lakes. Our data raise key questions about the processes that trigger the tremendous variability in biota and GM-MIB production in the SLR, the subject of our continued research.

Phytoplankton fuels Delta food web

Populations of certain fishes and invertebrates in the Sacramento-San Joaquin Delta have declined in abundance in recent decades and there is evidence that food supply is partly responsible. While many sources of organic matter in the Delta could be supporting fish populations indirectly through the food web (including aquatic vegetation and decaying organic matter from agricultural drainage), a careful accounting shows that phytoplankton is the dominant food source. Phytoplankton, communities of microscopic free-floating algae, are the most important food source on a Delta-wide scale when both food quantity and quality are taken into account. These microscopic algae have declined since the late 1960s. Fertilizer and pesticide runoff do not appear to be playing a direct role in long-term phytoplankton changes; rather, species invasions, increasing water transparency and fluctuations in water transport are responsible. Although the potential toxicity of herbicides and pesticides to plankton in the Delta is well documented, the ecological significance remains speculative. Nutrient inputs from agricultural runoff at current levels, in combination with increasing transparency, could result in harmful algal blooms.

Indirect effect of different fish communities on nutrient chlorophyll relationship in shallow hypertrophic water quality reservoirs

Effects of different fish communities on the proportion of different nitrogen and phosphorous forms and the amount of phytoplankton (chlorophyll a) were examined in two consecutive years (1992–1993) in three Hungarian shallow water reservoirs (Cassette and outer reservoir of the Kis–Balaton Water Protection System, and Marcali reservoir). Possible interactions between nutrient concentrations and the amount of phytoplankton in these reservoirs were also examined. Considerable differences in the proportions of different nutrient forms were observed between the three test sites, which could be explained by the presence of different fish stocks in these reservoirs. In the Cassette, the fish biomass necessary for a water quality improvement was around 50 kg ha−1. Phytoplankton biomass was controlled by the zooplankton, consequently chlorophyll a concentrations decreased considerably, while those of dissolved nutrients significantly increased. In the outer reservoir, phytoplankton was controlled bottom-up, since the 250 kg ha−1 fish biomass was larger than the critical value due to the high proportion of planktivorous species. Chlorophyll a concentrations were high, and nutrients were mainly in particulate form (in algal cells). In the Marcali reservoir, the recently introduced silver carp population could not control fully the phytoplankton. The biomass of phytoplankton decreased only slightly, while its composition changed considerably. Although biomanipulation with silver carp is suitable for ceasing cyanobacterial blooms, reduction of the amount of planktivorous fish seems to be a more adequate method for increasing water transparency, rather than introduction of phytoplankton feeding fish.

The relation between sediment nutrient content and macrophyte biomass and community structure along a water transparency gradient among lakes of the Mackenzie Delta

Macrophyte abundance and distribution among lakes of the Mackenzie Delta were assessed where increasing distance from the river (chain set) and increasing frequency of flooding (sill set) corresponded with increasing water transparency. Overall, sediment organic matter (OM) and total nitrogen (TN) content increased with increasing biomass of macrophytes but was higher in the sill set than in the chain set. The amount of phosphorus (P) in sediments was similar among lakes, but pore-water P was appreciably higher in the chain set. Increasing sediment OM and water clarity corresponded with increasing biomass of macrophytes in the lakes. Community structure shifted from dominance by erect Potamogeton at low and intermediate transparency and moderate sediment OM content to low-growing Chara and Ceratophyllum at high transparency and high sediment OM. Similar transparency in the chain set supported greater biomass of macrophytes than in the sill set. A high rate of inorganic sedimentation (linked with frequent flooding) and organic sedimentation (linked with high transparency and plant biomass) may result in the most suitable substrate for the growth of macrophytes among lakes of the Mackenzie Delta. Submersed plant biomass was higher in the Mackenzie Delta lakes than in temperate lakes and comparable to that in the temperate and tropical floodplains, despite the high-latitude location.

Increased photoreactivity of DOC by acidification: Implications for the carbon cycle in humic lakes

Effects of ultraviolet (UV)‐B radiation and acidification on pelagic carbon flux in a humic lake (dissolved organic carbon [DOC] ~15 mg C L−1) were studied in a mesocosm experiment during the summer of 2000. Triplicate tanks (107 liters volume, 1 m high) were exposed to natural solar radiation, a daily extra dose of UV‐B radiation, or kept dark. One set of tanks was submitted to a decrease in pH (from 5.7 to 4.7), and one set was kept at the natural pH level. During 70 d, water samples were taken regularly from the mesocosms for measurements of DOC, absorbance, dissolved inorganic carbon (DIC), and pH. Additionally, we regularly incubated samples to measure photooxidation rates, primary production, and community respiration. We found an increase in the photooxidation rates in the acidified mesocosms relative to ambient pH. The greater abiotic production of DIC (i.e., photooxidation) in acidified conditions could explain ~27% of the decline in DOC in the same conditions. Laboratory experi ments were done to test the effects of pH on the dissolved organic matter (DOM) photoreactivity. At lower pH values, we found both higher abiotic DIC production and specific absorbance fading, relative to neutral pH values in water from a humic lake. In a separate experiment, samples were exposed to prolonged irradiation under laboratory conditions, resulting in complete loss of absorptivity in the wavelengths between 290 and 400 nm. Decreases in DOC in the long‐term exposure caused by photochemical mineralization were ~45 and 55% of the initial pool for natural pH and acidified samples, respectively, at the end of the experiment. An increase in the dissolved organic matter photoreactivity by acidification could be an important mechanism to explain the increased water transparency and in‐lake DOC removal in acid lakes found in several previous studies.

The role of charophytes in increasing water transparency: a case study of two shallow lakes in Estonia

The hydrochemical regime and the biota in two lakes of Vooremaa landscape protection area, Central Estonia, were studied in 2000–2001 within the frames of the EC project ECOFRAME aimed to work out water quality criteria for shallow lakes in Europe. Lake Prossa is a macrophyte-dominated lake with an area of 33 ha and a mean depth of 2.2 m. Most of the bottom in this lake is covered by a thick mat of charophytes all the year round. Lake Kaiavere located at a distance of 10 km from the former is a much larger (250 ha, mean depth 2.8 m) plankton-dominated lake. Despite the different area and the Chara meadows in Lake Prossa, the nutrient dynamics were very similar in both lakes. The first vernal phytoplankton peak was expressed in reduced Secchi depth in both lakes. After that peak, the water became clear in L. Prossa but remained turbid in Lake Kaiavere. In the Chara-lake the mean individual weight of zooplankton reached its maximum in both years in June. In the year 2000 it coincided with the peak of the zooplankton/phytoplankton ratio and with the minimum of soluble reactive phosphorus both contributing to the initiation of the clear-water period. Towards autumn, the individual mean weight of zooplankton decreased in the Chara-lake and remained smaller than in the plankton-dominated lake. Hence, zooplankton grazing could initiate the clear-water phase in the Chara-lake but other factors were needed for its maintenance. Another parameter that showed a clear difference between these two lakes was the carbonate alkalinity. It was rather stable or even increasing during spring in the plankton-dominated lake, while it decreased by nearly 50% between April and July in the Chara-lake. The calculated saturation level of calcite remained still lower in the Chara-lake showing that the co-precipitation of algae with autochthonous calcite could not explain the observed increase in water transparency above Chara meadows. The reduced sediment resuspension and the possible allelopathic influence of charophytes on phytoplankton remain the main explanations for the maitenance of the extensive clear-water period in the Chara-lake.

The influence of water depth and flow regime on phytoplankton biomass and community structure in a shallow, lowland river

The taxonomic composition and biomass of phytoplankton in the San Joaquin River, California, were examined in relation to water depth, flow regime, and water chemistry. Without substantial tributary inflow, maintenance demands exceeded algal production during summer and autumn in this eutrophic, `lowland type' river due to light-limiting conditions for algal growth. Streamflow from tributaries that drain the Sierra Nevada contributed to a substantial net gain in algal production during the spring and summer by increasing water transparency and the extent of turbulence. Abundances of the major taxa (centric diatoms, pennate diatoms and chlorophytes) indicated differing responses to the longitudinal variation in water depth and flow regime, with the areal extent of pools and other geomorphic features that influence time-for-development being a major contributing factor to the selection of species. Tychoplanktonic species were most abundant upstream and in tributaries that drain the San Joaquin Valley. Seasonally-varying factors such as water temperature that influence algal growth rates also contributed significantly to the selection of species. Nutrient limitation appears not to be a primary constraint on species selection in the phytoplankton of this river.

Differences in the exploitation of bream in three shallow lake systems and their relation to water quality

SUMMARY 1. The development of bream populations, water transparency, chlorophyll‐a concentration, extent of submerged vegetation and densities of the zebra mussel, Dreissena polymorpha, were analysed in three shallow eutrophic lake systems subject to different fish management.2. In Lake Veluwemeer, the bream population was reduced from c. 100 to 20 kg ha−1 after 5 years of fishing. The mortality caused by the fishery was estimated at 38% of bream &gt;15 cm in addition to a 13% natural mortality of bream &gt;17 cm. The decline was followed by an expansion of the Chara beds present in the shallow parts, an increase in water transparency in the open‐water zone, an increase in the density of zebra mussels and a decrease in chlorophyll‐a concentrations.3. The newly created Lake Volkerak showed trends opposite to those in Lake Veluwemeer. Bream colonised the lake in 1988 and reached a biomass of c. 140 kg ha−1 in 1998. The water transparency decreased from a maximum of 3 m to c. 1 m and the chlorophyll‐a concentration increased from 5 to 45 μg L−1. Submerged vegetation colonised up to 20% of the total lake area in the first 5 years after creation of the lake in 1987 but decreased to 10% as turbidity increased.4. Seine fishery in the Frisian lake system did not appear to affect the bream population despite annual catches as high as 40–50 kg ha−1. The estimated natural mortality of fish &gt;15 cm was 15% and mortality by fishery was 26%. The high loss was apparently compensated by good recruitment and high growth rates resulting from a c. 1 °C higher water temperature during the years when bream were removed by fishing. There was only a slight decrease in chlorophyll‐a concentrations and a slight increase in water transparency.5. The results of this study suggest that the effects of bream exploitation in eutrophic lakes can vary depending on the efficiency of the fishery, recruitment success and temperature regime. In the absence of fishery, bream dominated the fish community in the study lakes and apparently prevented D. polymorpha and submerged vegetation from establishing because of physical disturbance, enhanced internal P‐loading and resettling of resuspended sediments.

The influence of water transparency on the distribution and abundance of macrophytes among lakes of the Mackenzie Delta, Western Canadian Arctic

SUMMARY 1. Macrophyte abundance and distribution was assessed in a chain of six interconnected lakes (all with the same flooding frequency) in the Arctic, where increasing distance from the Mackenzie River channel resulted in a gradient of water transparency (‘chain‐set’ lakes), and in a group of 26 spatially discrete lakes where increasing frequency and duration of lake flooding with river water (controlled by sill height) also resulted in a transparency gradient (‘sill‐set’ lakes).2. Among the chain‐set lakes, above‐ground macrophyte biomass increased from 0 to 1000 g m−2 with increasing water transparency. Among the sill‐set lakes, the transparency gradient among the lakes was less well defined and the relations with biomass were more varied. A decrease in flooding was associated with increasing water transparency and an increasing biomass of macrophytes from about 0 to over 2000 g m−2. For a specific flood frequency, however, the effect of flooding was much greater when lakes were directly connected to a river channel than when floodwaters flowed first through an intervening lake. Among infrequently flooded lakes the effect of flooding on water transparency and biomass was negligible.3. Among relatively clear lakes in both sets of lakes, biomass increased with increasing water transparency and decreasing lake depth. Among relatively turbid lakes, however, biomass increased with the combined effect of increasing water colour (decreasing water transparency) and increasing lake water depth. The increases in biomass with increasing water colour (coloured dissolved organic matter) and increasing depth, which together result in reduced light at the bed, may be explained by reduced exposure to ultra violet light.4. An average light attenuation of 1.3 m−1 (Secchi depth about 1 m) over the growing season appears to represent a threshold water transparency which, in combination with water depths early in the growing season, is consistent with a light supply on the bed required for growth of the common macrophytes in lakes of the Mackenzie Delta. However, a comparison with other systems indicates that macrophytes among lakes of the Mackenzie Delta grow deeper, for a given level of transparency, than is reported in lakes at lower latitude, despite the lower sun angles and increased reflectivity of water surfaces in the arctic.5. A complete accounting of water transparency (at PAR and UV wavelengths), lake depth, summer sun angle and duration of sunlight may be necessary to explain patterns of macrophyte growth among lakes across a full range of latitudes.

Annual primary production: Patterns and mechanisms of change in a nutrient‐rich tidal ecosystem

Although nutrient supply often underlies long‐term changes in aquatic primary production, other regulatory processes can be important. The Sacramento‐San Joaquin River Delta, a complex of tidal waterways forming the landward portion of the San Francisco Estuary, has ample nutrient supplies, enabling us to examine alternate regulatory mechanisms over a 21‐yr period. Delta‐wide primary productivity was reconstructed from historical water quality data for 1975–1995. Annual primary production averaged 70 g C m−2, but it varied by over a factor of five among years. At least four processes contributed to this variability: (1) invasion of the clam Potamocorbula amurensis led to a persistent decrease in phytoplankton biomass (chlorophyll a) after 1986; (2) a long‐term decline in total suspended solids—probably at least partly because of upstream dam construction—increased water transparency and phytoplankton growth rate; (3) river inflow, reflecting climate variability, affected biomass through fluctuations in flushing and growth rates through fluctuations in total suspended solids; and (4) an additional pathway manifesting as a long‐term decline in winter phytoplankton biomass has been identified, but its genesis is uncertain. Overall, the Delta lost 43% in annual primary production during the period. Given the evidence for food limitation of primary consumers, these findings provide a partial explanation for widespread Delta species declines over the past few decades. Turbid nutrient‐rich systems such as the Delta may be inherently more variable than other tidal systems because certain compensatory processes are absent. Comparisons among systems, however, can be tenuous because conclusions about the magnitude and mechanisms of variability are dependent on length of data record.

Nutrient and grazing factors in relation to phytoplankton level in a eutrophic shallow lake: the effect of low macrophyte abundance

The ability of submerged macrophytes to moderate the structure of food webs with respect to lake eutrophication management has been intensively studied in recent years. Many lake managers have adopted the option of increasing macrophyte abundance in order to restore eutrophic waters, with a view to improve water quality, increase water transparency and reduce phytoplankton biomass. These studies are mostly based upon the comparison of macrophyte-rich and macrophyte-free waters. There is less quantitative information on how different levels of macrophyte abundance, in particular very low levels, influence ecosystem structure, or vice versa, within the same ecosystem. This paper takes a multivariate statistical approach to examine ecosystem function with low macrophyte abundance (ca. 5%) in the context of nutrient–phytoplankton–zooplankton interaction in a eutrophic shallow lake. It shows that with low levels of macrophyte coverage, phytoplankton biomass was only limited by nutrient (TP and Si) availability, whilst nutrient (Si) as well as grazing (zooplankton and Daphnia) factors were both limiting phytoplankton levels in macrophyte-free waters. With respect to interactions between Daphnia and chlorophyll-a, the results suggest that low macrophyte cover does not result in a reduction of phytoplankton biomass due to the presence of Daphnia. Rather, low macrophyte cover provides a refuge that maintains constant Daphnia predation, so reducing chlorophyll-a fluctuation. These results add credence to the causal interpretation of different interactions amongst nutrients, phytoplankton and zooplankton at very low macrophyte densities.