Large Shallow Eutrophic Lake Research Articles

Wetlands as principal zones of methylmercury production in southern Louisiana and the Gulf of Mexico region

It is widely recognized that wetlands, especially those rich in organic matter and receiving appreciable atmospheric mercury (Hg) inputs, are important sites of methylmercury (MeHg) production. Extensive wetlands in the southeastern United States have many ecosystem attributes ideal for promoting high MeHg production rates; however, relatively few mercury cycling studies have been conducted in these environments. We conducted a landscape scale study examining Hg cycling in coastal Louisiana (USA) including four field trips conducted between August 2003 and May 2005. Sites were chosen to represent different ecosystem types, including: a large shallow eutrophic estuarine lake (Lake Pontchartrain), three rivers draining into the lake, a cypress-tupelo dominated freshwater swamp, and six emergent marshes ranging from a freshwater marsh dominated by Panicum hemitomon to a Spartina alterniflora dominated salt marsh close to the Gulf of Mexico. We measured MeHg and total Hg (THg) concentrations, and ancillary chemical characteristics, in whole and filtered surface water, and filtered porewater. Overall, MeHg concentrations were greatest in surface water of freshwater wetlands and lowest in the profundal (non-vegetated) regions of the lake and river mainstems. Concentrations of THg and MeHg in filtered surface water were positively correlated with the highly reactive, aromatic (hydrophobic organic acid) fraction of dissolved organic carbon (DOC). These results suggest that DOC plays an important role in promoting the mobility, transport and bioavailability of inorganic Hg in these environments. Further, elevated porewater concentrations in marine and brackish wetlands suggest coastal wetlands along the Gulf Coast are key sites for MeHg production and may be a principal source of MeHg to foodwebs in the Gulf of Mexico. Examining the relationships among MeHg, THg, and DOC across these multiple landscape types is a first step in evaluating possible links between key zones for Hg(II)-methylation and the bioaccumulation of mercury in the biota inhabiting the Gulf of Mexico region.

A comparison of zooplankton densities and biomass in Lakes Peipsi and Võrtsjärv (Estonia): rotifers and crustaceans versus ciliates

The abundance and biomass of ciliates, rotifers, cladocerans and copepods were studied in Lake Peipsi and Lake Vortsjarv, both of which are shallow, turbid and large. Our hypothesis was that in a large shallow eutrophic lake, the ciliates could be the most important zooplankton group. The mean metazooplankton biomass was higher in Peipsi than in Vortsjarv (mean values and SD, 1.8 ± 0.7 and 1.3 ± 0.6 mg WM l−1). In Peipsi, the metazooplankton biomass was dominated by filtrators that feed on large-sized phytoplankton and are characteristic of oligo-mesotrophic waters. In Vortsjarv, the metazooplankton was dominated by species characteristic of eutrophic waters. The planktonic ciliates in both lakes were dominated by oligotrichs. The biomass of ciliates was much greater in Vortsjarv (mean 2.3 ± 1.4 mg WM l−1) than in Peipsi (0.1 ± 0.08 mg WM l−1). Ciliates formed about 60% of the total zooplankton biomass in Vortsjarv but only 6% in Peipsi. Thus, the food chains in the two lakes differ: a grazing food chain in Peipsi and a detrital food-chain in Vortsjarv. Consequently, top-down control of phytoplankton can be assumed to be much more important in Peipsi than in Vortsjarv. When the detrital food chain prevails, the planktonic ciliates become the most important zooplankton group in shallow, eutrophic and large lake. Neglecting protozooplankton can result in serious underestimates of total zooplankton biomass since two-thirds of the zooplankton biomass in Vortsjarv comprises ciliates.

Nitrogen dynamics in large shallow eutrophic Lake Chaohu, China

Temporal and spatial dynamics of nitrogen in lake and interstitial water were studied monthly in a large shallow, eutrophic lake in subtropical China from October 2002 to September 2003. The distribution of nitrogen was consistent with the idea that high nitrogen concentrations in the western part of the lake resulted from high levels of the nutrients from the surrounding cities through sewage-drainage systems. Nitrate was the predominant form of nitrogen in the overlying water, while ammonium was predominant in the interstitial water, indicating that strong oxidative nutrient regeneration occurred near the sediment-water interface. Nitrate could be an important dissolved inorganic matter source for phytoplankton, which in turn influenced the seasonal variations of nitrate concentrations in lake water. Significant positive correlation between ammonium fluxes and water temperature was observed and could probably be attributed to the intensified ammonification and nitrate reduction with increased temperature. Positive correlation between ammonium fluxes and algae biomass and Chl a concentrations may indicate that phytoplankton was an important factor driving ammonium fluxes in our study lake, and vice versa that higher fluxes of ammonium supported a higher biomass of the phytoplankton.

Metals in Surface Sediments of Large Shallow Eutrophic Lake Chaohu, China

Metal contents of surface sediments were analyzed temporally and spatially in Lake Chaohu, China. No obvious temporal variations were observed, which probably due to physio- and bio- mixing, e.g. wind and microbes, in this lake. Enrichment factor of some metals were generally greater than 1.0, suggesting significant anthropogenic impact on metal levels. Significantly positive correlations between concentrations of nutrient and metals indicated that the nutrients transported to this lake contributed, to some extent, to the enrichment of metals. The correlation between trace metals concentrations indicated the co-contamination of anthropogenically derived metal enrichment in surface sediment of Lake Chaohu.

Intra-habitat heterogeneity of environmental factors regulating bacterioplankton community composition in Lake Taihu, China

To investigate the intra-habitat heterogeneity of environmental factors that may have an impact on the bacterioplankton community composition (BCC), a 1 yr observation was done at 2 study sites in Lake Taihu, a large shallow eutrophic lake in China. One site was located at the center of Meiliang Bay, the other site was near the central region of the lake. Denaturing gradient gel elec- trophoresis (DGGE) of PCR-amplified partial 16S rRNA gene fragments was applied to analyze BCC. The unweighted pair group method with arithmetic mean (UPGMA) was used to investigate differ- ences among BCC. Redundancy analyses (RDA) of the gel patterns, in relation to the physical, chem- ical, and biological factors of the 2 sites, were performed. Differences between the samples in Mei- liang Bay and the lake center were evident during all periods. Four variables (the biomass of Microcystis spp., chl a, dissolved inorganic phosphorus (DIP), and water turbidity) were most strongly correlated with the bacterioplankton DGGE patterns. Particularly, significant intra-habitat hetero- geneity of the environmental factors was detected. In Meiliang Bay, seasonal variations in DIP con- centration had a profound impact on the composition of bacterioplankton communities, while in the lake center, wind-induced frequent sediment resuspension was an important reason for the varia- tions in BCC. The inherent intra-habitat heterogeneity of environmental conditions is essential to maintain the heterogeneity of BCC at the 2 study sites.

Intra-habitat heterogeneity of microbial food web structure under the regime of eutrophication and sediment resuspension in the large subtropical shallow Lake Taihu, China

Planktonic microbial community structure and classical food web were investigated in the large shallow eutrophic Lake Taihu (2338 km2, mean depth 1.9 m) located in subtropical Southeast China. The water column of the lake was sampled biweekly at two sites located 22 km apart over a period of twelve month. Site 1 is under the regime of heavy eutrophication while Site 2 is governed by wind-driven sediment resuspension. Within-lake comparison indicates that phosphorus enrichment resulted in increased abundance of microbial components. However, the coupling between total phosphorus and abundance of microbial components was different between the two sites. Much stronger coupling was observed at Site 1 than at Site 2. The weak coupling at Site 2 was mainly caused by strong sediment resuspension, which limited growth of phytoplankton and, consequently, growth of bacterioplankton and other microbial components. High percentages of attached bacteria, which were strongly correlated with the biomass of phytoplankton, especially Microcystis spp., were found at Site 1 during summer and early autumn, but no such correlation was observed at Site 2. This potentially leads to differences in carbon flow through microbial food web at different locations. Overall, significant heterogeneity of microbial food web structure between the two sites was observed. Site-specific differences in nutrient enrichment (i.e. nitrogen and phosphorus) and sediment resuspension were identified as driving forces of the observed intra-habitat differences in food web structure.

Studies on Temporal and Spatial Variations of Phytoplankton in Lake Chaohu

AbstractTemporal and spatial variations of the phytoplankton assemblage in Lake Chaohu, a large shallow eutrophic lake in China, were studied from September 2002 to August 2003. A total of 191 phytoplankton species was identified, among which Chlorophytes (101) ranked the first, followed by Cyanophytes (46) and Bacillariophytes (28). On average over the entire lake, the maximum total algal biomass appeared in June (19.70 mg/L) with a minimum (5.05 mg/ L) in November. In terms of annual mean biomass, cyanobacteria contributed 45.43% to total algal biomass, followed by Chlorophytes (27.14%), and Bacillariophytes (20.6%). When nitrate (NO3‐N) and ammonium (NH4‐N) concentrations dropped in spring, fixing‐nitrogen cyanobacterium (Anabaena) developed quickly and ranked the first in terms of biomass in summer. It is likely that dominance of zooplanktivorous fish and small crustacean zooplankton favored the development of the inedible filamentous or colony forming cyanobacteria. The persistent dominance of cyanobacteria throughout all seasons may indicate a new tendency of the response of phytoplankton to eutrophication in Lake Chaohu.

Multivariate Analysis of Interactions Between Phytoplankton Biomass and Environmental Variables in Taihu Lake, China

Phytoplankton variation in large shallow eutrophic lakes is characterized by high spatial and temporal heterogenity. Understanding the pattern of phytoplankton variation and the relationships between it and environmental variables can contribute to eutrophic lakes management. In this study Taihu Lake, one of the largest eutrophic fresh water lake in China, was taken as study area. The water body of Taihu Lake was divided into five regions viz. Wuli bay (WB), Meilian Bay (MB), West Taihu Lake (WTL), Main Body of Taihu Lake (MBTL) and East Taihu Lake (ETL). Concentrations of chlorophyll-a and the related environmental variables were determined in each region in the period 2000-2003. Factor analysis and multivariate analysis were applied to evaluate the interactions between phytoplankton variation and environmental variables. Results showed that the highest average concentrations of TN, TP and Chl-a were observed in WB, followed in a descending order by MB and WTL, and the lowest concentrations of TN, TP and Chl-a were observed in MBTL and ETL. Chl-a and TP concentrations in most regions (except ETL) declined during the study period. It suggested that to some extent the lake was recovering from eutrophication. However, persistent ascending of TN and NH(4)-N in all five regions indicated the deteriorating of water quality in the study period. Results of multivariate showed that the relationships between phytoplankton biomass and environmental variables varied among regions. TP illustrated itself a controlling role on phytoplankton in WB, MB, WTL and MBTL according to the significant positive relations to phytoplankton biomass in these regions. Nitrogen could be identified as a limiting factor to phytoplankton biomass in ETL in view of the positive correlations between TN and phytoplankton and between NH(4)-N and phytoplankton. Spatial variation of interactions between phytoplankton and environmental parameters suggested proper eutrophication control measures were needed to restore ecological system in each region of Taihu Lake.

Spatiotemporal variations of internal P-loading and the related mechanisms in the large shallow Lake Chaohu

Spatiotemporal variations of P species and adsorption behavior in water column, interstitial water, and sediments were investigated in the large shallow eutrophic Lake Chaohu. Orthophosphate (Ortho-P) and total phosphorus (TP) concentrations were significantly higher in the western part than in the eastern part of the lake, due to different nutrient inputs from the surrounding rivers. Moreover, particulate phosphorus (PP) concentration was in a similar spatial pattern to Ortho-P and TIP concentrations, and also showed significantly positive correlation with the biomass of Microcystis, indicating more uptake and store of phosphorus by Microcystis than by other algae. Increase of pH and intensive utilization of P by phytoplankton were the main factors promoting P (especially Fe-P) release from the sediment to interstitial water during the cyanobacterial blooms in Lake Chaohu. Spatial dynamics in TP concentration, P species and adsorption behavior of the sediment, coupled with the statistical analyses, suggested that the spatial heterogeneity of P contents in the sediment was influenced by various factors, e.g. human activities, soil geochemistry and mineral composition. In spite of similar TP contents in the sediments, increase in proportion of Fe-P concentration in the sediment may result in a high risk of P release.

Evaluating genotoxicity associated with microcystin-LR and its risk to source water safety in Meiliang Bay, Taihu Lake

In recent years cyanobacteria blooms have become a severe problem in Taihu Lake, a large shallow eutrophic lake in China. Microcystins produced by certain genera of cyanobacteria can affect public health in this area because of their acute and chronic toxic effects. In this study, samples of cyanobacteria were collected and extracted by two solvent systems. The extracts were tested with three short-term genotoxicity assays, the ara test, the Ames test, and the SOS/umu test. In addition, temporal variation in the concentrations of microcystin-LR in the water samples was determined and monitored by an ELISA assay. Then the concentration of microcystin-LR in the drinking water was estimated. The risk of microcystin-LR exposure by drinking water was assessed according to tolerable daily intake (TDI). The three genotoxicity assays showed negative results regardless of the solvent system used, and there were clear inconsistencies in the spatiotemporal profiles of genotoxic potential and microcystin concentrations in Taihu Lake. Risk assessment showed that the drinking water from Taihu Lake was not safe from the end of July to the beginning of November because of a high concentration of microcystin-LR. Our study indicated the drinking water from Taihu Lake posed a risk because of the microcystin-LR, although it was neither genotoxic nor associated with genotoxicity of the lake water.

Phytoplankton response to changed nutrient level in Lake Peipsi (Estonia) in 1992–2001

In the 1990s, the nutrient load from the catchment area, carried into the large shallow eutrophic Lake Peipsi (area 3555 km 2 ), diminished in comparison with the 1980s. The aim of the investigation was to analyse statistically biotic response to alterations in nutrient level in the lake following the reduction in external loading. In-lake changes of the relationship between phosphorus and nitrogen compounds and phytoplankton were analysed for 10 years (1992–2001). The content of biogenic elements (total phosphorus, orthophosphate, total nitrogen and ammonia) indicated a decrease and thereafter a stabilization of the parameter values during the observation period. An opposite trend was observed for the nitrite and nitrate ions. The lowest Ntot:Ptot ratio (about 11–13) was registered in 1995–96. However, despite a definite decrease of the nutrient level, the biomass of phytoplankton (particularly cyanobacteria) displayed an increasing trend. Strong and long-lasting (up to October–November) algal blooms were noted in recent years. The structure of the phytoplankton community changed considerably: the share of cyanobacteria (particularly, heterocystous forms) demanding less nutrients increased while the contribution of chlorophytes decreased. The amount of nutrients per biomass unit of different phytoplankton groups decreased. Water level appears to be a significant factor affecting the content of dissolved inorganic nutrients. In particular, NO3N increased and PO4P decreased at higher water level. Large inter-annual water level fluctuations will decrease stability of the ecosystem.

Long-term trends in the fish communities of Lakes Peipsi and Võrtsjärv (Estonia)

This study was undertaken to determine the direction and extent of time related changes in the fish communities of the large shallow eutrophic lakes Peipsi and Võrtsjärv. The official statistics of commercial fisheries for the periods 1931–1940 and 1950–1999 were used for trend analysis. In the ichthyocoenosis of both lakes, several drastic and economically important changes have occurred during recent decades. Some trends in the population dynamics of fish (e.g., sharp decline of the stock of vendace Coregonus albula (L.) in Lake Peipsi) are probably irreversible owing to the eutrophication of the lake. A sharp decrease in the stock of smelt Osmerus eperlanus (L.), caused by nighttime water anoxia in the summer of several years, was followed by a quick recovery. Periodic changes in the population of pike Esox lucius L., are related to water level fluctuations. The population dynamics of several fishes, particularly pikeperch Stizostedion lucioperca (L.), are, to a large extent, controlled by fishery policy, although climatic factors are also of importance. The state of the stock of eel Anguilla anguilla (L.), depends entirely on introduction of young eels.

Plankton Seasonal Dynamics and Its Controlling Factors in Shallow Polymictic Eutrophic Lake Võrtsjärv, Estonia

An analysis of plankton seasonal succession in large shallow eutrophic lake Vortsjarv (270 km 2 , mean depth 2.8 m, max. depth 6 m) is presented. Weekly samples for 1995 have been analysed using the PEG model approach. In winter, light was the main factor controlling phytoplankton growth. In early spring phytoplankton was mainly resource-controlled, competition for phosphorus being the main driving force. Ciliates (Paradileptus sp., Strobilidium sp. and Vorticella sp.) were the first herbivores which started to increase in April causing a twofold decline of phytoplankton biomass. The annual maximum of primary production (PP) in early May was probably caused by soluble reactive phosphorus (SRP), regenerated by herbivores, and stirred up from bottom sediments as a result of strong wind stress. This primary production peak provided substrate for further increase of bacterial biomass. The more abundant food supply supported the development of the second spring peak of herbivores (ciliates, rotifers, juvenile copepods) which was followed by the second modest clear water phase in late May. Silicon was depleted by the end of May causing a strong decrease in primary production of the diatom-dominated community. whereas the biomass of the cyanophytes increased under the improved nutrient conditions. Some weeks later, inorganic N was depleted and the period of N limitation with the appearance of N-fixing cyanophytes began. The ciliate collapse at the beginning of June coincided with the start of the cladoceran development and with the increase of other metazooplankton groups. This explains the further decrcase of the biomass of phytoplankton and bacteria in spite of their high production. Beginning from late June, silicon appeared again and SRP started to occur periodically. while inorganic N remained close to zero until November. During this period, phytoplankton development relied to a great extent on the N-fixation and N-regeneration potential. The collapse of the ciliate community in September removed the top-down control from bacteria and their biomass increased, while the development of cladocerans still suppressed phytoplankton biomass in spite of a quite high PP. In October phytoplankton biomass and chlorophyll a (Chla) increased, SRP was completely depleted by the middle of October reflecting a slow regeneration due to the declined activity of zooplankton in cold water. In November nitrates appeared again. and silicon reached the same level as in spring. The biomass of N-fixing Aphanizomenon skujae decreased while Limnothrix redekei and L. planctonica were quite abundant together with diatoms.

Perspectives for submerged macrophytes in shallow lake restoration projects in The Netherlands

Some well-documented studies on restoring eutrophic lake systems in The Netherlands by fish stock management have been evaluated with the emphasis on the role of macrophytes. Furthermore, the factors determining the light climate for submerged macrophytes in a large shallow eutrophic lake (Lake Veluwe) have been assessed and the potential success of biomanipulation in large scale projects is discussed. Today relatively little attention has been paid to macrophyte management although the importance of macrophytes in lake restoration has been recognized regularly. The biomanipulation strategy was successful in small scale projects. In a large scale project, however, wind-induced resuspension may largely determine the underwater light climate through attenuation by the water column and periphytic layer. Therefore, restoration of relatively large waterbodies by fish stock management only is expected not to lead to any noteworthy improvement of the light climate for submerged macrophytes. Additional measures aimed at reducing wind-induced resuspension of sediment particles and reestablishing of the macrophyte stands are required for successful biomanipulation strategies. Water quality managers should pay more attention to macrophyte stands in biomanipulation projects because macrophytes enhance a more stable and diverse ecosystem. Restoration objectives and the methods of their achievement must be carefully planned since an abundant submerged macrophyte vegetation may have undesirable effects as well.

洞爺湖臨湖実験所北海道大学水産学部付属実習施設

Hokkaido is a treasure land for limnology. There are many lakes of different types ; large oligotrophic deep caldera lakes, mesotrophic lakes with intermediate depth, large shallow eutrophic lakes, large brackish water lakes on the shore and also many dystrophic bog waters. Natural lakes excluding small ponds in Hokkaido amount to 96. In addition, 55 artificial dams were erected for the purpose of getting electric power and agricultural irrigation (Hokkaido Fish Hatchery 1969).Lake Toya approximately 11 km × 9 km (Figs. 1 and 2) is a large caldera lake located on the southwestern Hokkaido, with three islands in its center. Its maximum depth measures about 188 m. This lake is located in a comparatively mild climate area in Hokkaido with an annual variation of air temperature of 23.5 C to minus 3.2 °C (on decade average) (Fig. 3).The lake surface is never frozen except near the shore when winter is exceptionally cold. The temperature on the surface varies from 25 °C in August to about 4 °C in January-February. Thermocline develops from the middle of July, and is located at 5-20 m in August and disappears in late November. In recent years hydrogen ion concentration has been greatly increased due to the drainage from sulphur mining. This discharge has happened since 1939. The pH value was nearly neutral at the time before that discharge but it has gradually lowered year by year, in recent years the change was very rapid. It now measures pH 5-4 (1970) (Fig. 4 and Table 1). Along with this acidification of the water, dominant species of plankton altered (Table 2). Primary production during the period from August to November (1967-1968) was estimated as 40-150 mgC/m2/day.In 1893 eggs of land-locked form of sockeye salmon, Oncorhynchus nerka (Himemasu in Japanese name), were transferred from Lake Akan into this lake ; and in 1926 eggs of rainbow trout, Salmo gairdnerii irideus (Nijimasu in Japanese name), were also transferred from the Chitose Hatchery. In addition, eggs of pond smelt, Hypomesus olidus (Wakasagi in Japanese name), were sown for 5 years since 1926 from Kasumigaura and other lakes. These fishes have well propagated in this lake and much supported commercial and sport fisheries. Fig. 6 shows the number of fry of land-locked sockeyes released from the hatchery on the shore and the catch of grown fish per fishing effort during the fishing season (December-August). It is indicated that the year class released in 1963 did not grow well because the catch of grown fish after 5-6 years was very poor. It directly or indirectly derived from the acidification of lake water. In addition, catch of the pond smelt and a commercially useful shrimp, Palaemon paucidens, were almost negligible after 1965 (Fig. 6).Toya Limnological Station was established in 1937 taking over the Hatchery facilities operated by Fisheries Corporation (Figs. 9, 10 and 11). One of the writers, Wataru Otomo, Assistant Professor of Hokkaido University (formerly Hakodate College of Fisheries) has been a resident biologist for 34 years since 1937, and is retiring in March 1971 at the age of 63 (Fig. 7). Buildings of the station are old and the laboratory facilities are poor at present (Fig. 8). Every autumn, field operation and laboratory analysis in limnological observations and practice of artificial fertilization of land-locked sockeye salmon have been given to 40 students in biology course of the Faculty of Fisheries. Researches of visiting biologists were made at times. Limnological and productivity studies were conducted during 1966-1968 by a group participating in Japan International Biological Programme, Productivity of Fresh-water Communities.