Subtropical Eutrophic Lake Research Articles

Effects of Microcystis blooms on the crustacean plankton community: enclosure experiments in a subtropical lake

An enclosure experiment was conducted in July–September 2001 in subtropical eutrophic Lake Donghu (China) to test a hypothesis that a moderate cyanobacterial biomass would have a positive effect on small-sized cladocerans. Eight enclosures (12.5 m3) were arranged with different nutrient concentrations using the lake water, tap water, and sediment from Lake Donghu. Microcystis blooms appeared in enclosures with higher nutrient concentrations and the average fresh weight biomass of Microcystis spp. ranged from 4.6 to 30.4 mg l−1 during the bloom period. Three cladocerans (Moina micrura, Diaphanosoma brachyurum, and Ceriodaphniacornuta) and two cyclopoids (Mesocyclops dissimilis and Thermocyclops taihokuensis) dominated the crustacean plankton community during the experimental period. The C. cornuta biomass constituted the greatest percentage (55.9–90.0%) of cladoceran biomass in the Microcystis bloom treatments. When the Microcystis biomass increased, the average biomass of C. cornuta increased and the biomass of M. micrura and D. brachyurum decreased, whereas the cyclopoid biomass did not change significantly. The total biomass of cladoceran and crustacean plankton were significantly positively correlated with the Microcystis biomass. Our results indicate that a moderate biomass of Microcystis spp. can favor crustacean plankton to some extent and, furthermore, may impact food web structures in a eutrophic lake.

Compositional differences among planktonic ciliate communities in four subtropical eutrophic lakes in China

Ciliate community composition was investigated in four subtropical eutrophic lakes with cyanobacterial blooms in three seasons during 2009–2010 using the quantitative protargol staining approach. Among the four lakes, Lake Taihu and Chaohu are situated in the mid-lower Yangtze Plain near sea level, and Lake Dianchi and Xingyunhu are located on the Yunnan–Kweichow Plateau. A total of 102 ciliate species, representing 15 orders and 66 genera, were studied. Ciliate communities in Lake Taihu and Chaohu were characterized by abundant algivorous and mixotrophic oligotrichs and prostomatids, while Lake Dianchi and Xingyunhu ciliate communities were primarily characterized by voracious bacterivorous scuticociliates and peritrichids. Cluster analysis based on the transformed square root of ciliate abundance showed that samples from Lake Taihu and Chaohu composed one group whereas samples from Lake Dianchi and Xingyunhu composed another group in each season. Our results indicated that all dominant ciliate species are ubiquitous and that species dominance is regulated by different factors in different subtropical eutrophic lakes.

Zooplankton response to the lake restoration in the drinking-water source in Meiliang Bay of subtropical eutrophic Lake Taihu, China

A 3-year ecological engineering project was carried out from 2003 to 2005 in the drinking water source in Meiliang Bay, Lake Taihu, China. The project area was divided into three zones: Zone C (outside the area), Zone B (pelagic part within the area) and Zone A (littoral part within the area). The community composition, density and biomass of zooplankton were studied as a part in the lake restoration study. Also, the abundance, biomass and composition of fish in the engineering area were annually analyzed in the littoral zone (Zone A). The average density of cladocerans decreased from the outer part to the inner part of the lake, i.e., from Zone C to Zone A, while the average density of copepod did not significantly vary among the 3 zones. The average densities of rotifer and ciliate increased from Zone C to Zone A. RDA analysis indicated that Microcystis biomass more impacted the zooplankton compositions after temperature. In Zone A, many planktivorous fishes, e.g., Coilia nasus, Hemiculter leucisculus, Pseudorasbora parva, dominated the fish community, which caused a marked decrease in the abundance of cladocerans, especially large-sized cladoceran Daphnia galeata and Daphnia obtusa. Our results suggested that both fish predation and other environmental factors such as the Microcystis bloom greatly affected the zooplankton community during the restoration of subtropical eutrophic lakes.

Low carbon dioxide partial pressure in a productive subtropical lake

The purposes of this study were to assess if Lake Apopka (FL, USA) was autotrophic or heterotrophic based on the partial pressure of dissolved carbon dioxide (pCO2) in the surface water and to evaluate factors that influence the long-term changes in pCO2. Monthly average pH, alkalinity and other limnological variables collected between 1987 and 2006 were used to estimate dissolved inorganic carbon (DIC), pCO2 and CO2 flux between surface water and atmosphere. Results indicated that average pCO2 in the surface water was 196 μatm, well below the atmospheric pCO2. Direct measurements of DIC concentration on three sampling dates in 2009 also supported pCO2 undersaturation in Lake Apopka. Supersaturation in CO2 occurred in this lake in only 13% of the samples from the 20-year record. The surface-water pCO2 was inversely related to Chl a concentrations. Average annual CO2 flux was 28.2 g C m−2 year−1 from the atmosphere to the lake water and correlated significantly with Chl a concentration, indicating that biological carbon sequestration led to the low dissolved CO2 concentration. Low pCO2 and high invasion rates of atmospheric CO2 in Lake Apopka indicated persistent autotrophy. High rates of nutrient loading and primary production, a high buffering capacity, a lack of allochthonous loading of organic matter, and the dominance of a planktivorous–benthivorous fish food web have supported long-term net autotrophy in this shallow subtropical eutrophic lake. Our results also showed that lake restoration by the means of nutrient reduction resulted in significantly lower total phosphorus (TP) and Chl a concentrations, and higher pCO2.

Plankton biomass partitioning in a eutrophic subtropical lake: comparison with results from temperate lake ecosystems

Plankton were sampled for 6 years in a subtropical eutrophic lake in FL, USA, and absolute and relative carbon biomass was determined for bacteria, phytoplankton, heterotrophic and phototrophic nanoflagellates, ciliates, rotifers and crustacean zooplankton. We compared the results with findings from a comprehensive study of carbon biomass partitioning in eutrophic German lakes with elucidate common patterns and differences. Similarities between the temperate and subtropical systems included: similar seasonal dynamics, with maximal carbon biomass of nanoflagellates and metazoan zooplankton in spring and phytoplankton in summer to autumn, yearly averaged carbon occurring mainly in the phytoplankton and phytoplankton accounting for a much greater proportion of carbon than bacteria. There also were differences: the Florida lake had lower absolute and relative carbon biomass in crustacean zooplankton, stronger dominance of protozoa in total grazer carbon biomass, a lower ratio of zooplankton to phytoplankton carbon and almost a monoculture of predation-resistant copepods (versus a relatively balanced distribution of carbon among cladocerans, copepods and rotifers in the temperate lakes). The subtropical lake also had 4-fold higher relative biomass of small filamentous cyanobacteria in its phytoplankton, which we attribute to light limitation. Although the Florida and German studies did not measure biomass of planktivorous fish, the differences observed here are consistent with a recent hypothesis that fish predation exerts stronger top-down control on the pelagic food web in subtropical lakes than in temperate lakes of similar trophic status.

The Rotifer Assemblage in a Shallow, Eutrophic Chinese Lake and Its Relationships with Cyanobacterial Blooms and Crustacean Zooplankton

ABSTRACT Rotifer assemblage in the subtropical eutrophic Lake Chaohu was investigated monthly from September 2002 to August 2003. Forty-nine species belonging to 18 genera and 14 families were recorded. The highest densities of rotifer were observed during summer when there were heavy cyanobacterial blooms. There was a significant positive correlation between total rotifer density and the biomass of cyanobacteria. However, no correlations were found between the densities of rotifer and crustacean zooplankton, possibly owing to the paucity of large-bodied planktonic crustaceans. It is likely that the occurrence of cyanobacterial blooms not only caused the shift of dominant crustacean zooplankton from large species to smaller ones but also weakened the negative interaction between crustaceans and rotifers.

Influence of redox potential on phosphate-uptake by sediments in two sub-tropical eutrophic lakes

Biogeochemical reactions in shallow eutrophic lakes areaffected bythe changes in redox potential (Eh) as bottom sedimentsundergotemporal resuspension and settling. The stability of varioussediment P fractions and kinetics of P-uptake were evaluatedfortwo sub-tropical lakes (Lake Apopka and Lake Okeechobee,Florida)using sediment suspensions in closed systems maintained atvariousEh levels ranging from −235 to 555 mV. Redox potential hadminimal effect on the stability of NaOH-P (Fe-/Al-bound P plusmoderately resistant organic P) and HCl-P (Ca-/Mg-P) fractionsinLake Apopka sediments. Increases in ortho-P and NH4Cl-P(loosely-bound P plus labile organic P) concentrations wereobserved in highly reduced (Eh = −225 mV) Apopkasediments.Phosphate solubility diagrams and mineral equilibriacalculationssuggest that P-uptake by Apopka bottom sediments at elevated Pconcentrations (ortho-P ≤ 110 μM) was due toformationof Ca-P compounds and/or co-precipitation of P withCaCO3. Incontrast, the ortho-P concentrations for Lake Okeechobeebottomsediments increased exponentially with decreasing Eh. Thequantityof NaOH-P fractions for these sediments decreased withdecreasingEh, suggesting the release of Fe- and Mn-bound P intosolution.Phosphate-uptake by Okeechobee bottom sediments (pH 7.5,ambient)followed first order kinetics, yielding a rate constant (k)of 0.51 ± 0.05 h-1. Unlike that of Apopka, the mudsediments in Lake Okeechobee have strong affinity for P ineitheraerobic or anaerobic conditions. Results suggest that even incalcareous systems, Fe and Al, when present in highconcentrations(as in the case of Lake Okeechobee), are actively involved inregulating P-uptake and geochemistry.