Effects of zooplankton on nutrient availability and seston C : N : P stoichiometry in inshore waters of Lake Biwa, Japan

Forty-eight-hour experimental manipulations of zooplankton biomass were performed to examine the potential effects of zooplankton on nutrient availability and phytoplankton biomass (as measured by seston concentration) and C : N : P stoichiometry in eutrophic nearshore waters of Lake Biwa, Japan. Increasing zooplankton, both mixed-species communities and Daphnia alone, consistently reduced seston concentration, indicating that nearshore phytoplankton were generally edible. The zooplankton clearance rates of inshore phytoplankton were similar to rates measured previously for offshore phytoplankton. Increased zooplankton biomass led to increased concentrations of nutrients (NH₄-N, soluble reactive phosphorus [SRP]). Net release rates were higher than those found in previous measurements made offshore, reflecting the nutrient-rich nature of inshore seston. Zooplankton nutrient recycling consistently decreased TIN : SRP ratios (TIN = NH₄ + NO₃ + NO₂). This effect probably resulted from the low N : P ratios of nearshore seston, which were lower than those commonly found in crustacean zooplankton and thus resulted in low retention efficiency of P (relative to N) by the zooplankton. Thus, zooplankton grazing inshore may ameliorate algal blooms due to direct consumption but tends to create nutrient supply conditions with low N : P, potentially favoring cyanobacteria. In comparison with previous findings for offshore, it appears that potential zooplankton effects on phytoplankton and nutrient dynamics differ qualitatively in inshore and offshore regions of Lake Biwa. Received: September 4, 2000 / Accepted: January 23, 2001     

MACROALGAL TISSUE NUTRIENTS AS INDICATORS OF NITROGEN AND PHOSPHORUS STATUS IN THE FLORIDA KEYS

This study used the tremendous biochemical and ecological diversity of macroalgae to assess nitrogen and phosphorus availability at a broad, ecosystem‐level scale in the Florida Keys and nearby waters. Spatial variation in tissue nutrients (carbon, C; nitrogen, N; phosphorus, P) of dominant macroalgae were assessed, both as ratios and absolute values, along 12 inshore‐offshore transects in the Florida Keys and at 10 stations in nearby Florida Bay. The resulting detailed analysis demonstrated spatial and temporal patterns in macroalgal tissue nutrients. The transect data revealed no universal inshore‐offshore patterns in tissue nutrients and no obvious “hotspots” of nutrient enrichment. Similarly, when data were compared among segments, there was no universal geographical pattern in tissue nutrients for all species. The most striking result was that the N and P status of macroalgae in Florida Bay was significantly different than other locations. Macroalgae collected from Florida Bay generally had higher N and lower P levels than algae collected elsewhere. The most common inshore‐offshore pattern was higher %N and lower %P availability inshore; however, limited inshore‐offshore differences in N:P ratio suggests that both nutrients were generally readily available in proportional amounts required by the various species. Most species in this study had higher %N, and to a lesser extent, higher %P and %C in March than in July. Based on the published literature on other species of macroalgae, it appears that N and P are generally available in sufficient quantities that most macroalgal growth is not limited by either nutrient.     

Cost of predation avoidance in young-of-year lake trout (Salvelinus namaycush): growth differential in sub-optimal habitats

Selection of habitat to avoid predation may affect the diet of young-of-year (YOY) lake trout (Salvelinus namaycush). YOY lake trout may use inshore habitat to avoid predation; this habitat may be sub-optimal for growth. To test this, YOY lake trout were penned in nearshore and offshore pelagic areas of two arctic lakes. Toolik Lake had a lake trout population, the other lake, S6, did not. YOY lake trout in Toolik Lake lost weight, but those offshore lost less weight. The YOY lake trout in Lake S6 gained weight and those offshore gained more weight. The primary diet item of the YOY lake trout in both lakes during this experiment was the zooplankter Diaptomis probilofensis; it was also one of the most abundant species. However, its density inshore in Lake S6 was similar to inshore and offshore densities in Toolik Lake. The increased availability of alternative zooplankton prey in Lake S6 may account for the growth differential of YOY lake trout in Lake S6 relative to Toolik Lake. Bioenergetic modeling of YOY lake trout suggests that growth similar to that in the offshore of Lake S6 would be necessary for successful recruitment. If the reduced zooplankton availability in Toolik Lake leads to the reduced growth of YOY in the inshore and offshore pelagic areas, then these fish will be more susceptable to winter predation/starvation. For YOY lake trout to survive in Toolik Lake they most likely shift to feeding on benthic prey before the end of their first summer. Dept. of Chemical Engineering     

The inshore marine fishes of the Vestfold Hills region,Antarctica

Thirteen species of fish have so far been caught in the inshore waters around the Vestfold Hills, including the Rauer Islands, in depths down to approximately 100 m. Species caught depend markedly on the type of fishing gear used, but three species are clearly dominant numerically. Pagothenia bernacchii is most abundant in the shallower (< 20 m deep) weedy and rocky habitats, while Chionodraco hamatus is dominant in the deeper (> 20 m deep) nearshore troughs and further offshore. Pagothenia borchgrevinki occupies the specialized habitat associated with sea ice and close-inshore areas, including fjords and Burton Lake.The species list from the Vestfold Hills area is similar to lists from comparable locations in East Antarctica except for the major difference that C. hamatus has not yet been recorded from such shallow waters at the other locations, while P. bernacchii and P. hansoni are much more abundant in water deeper than 20 m at those sites than at Davis.     

New distribution record for the Asian tapeworm Bothriocephalus acheilognathi Yamaguti, 1934 in the Eastern Cape province,South Africa

Assuming that the inshore and offshore waters of Lake Victoria are impacted differently by human activities in its catchment, this study investigated the water quality dynamics of the lake. A total of 29 stations were sampled in 2005–2008 for dissolved oxygen (DO), pH, Secchi transparencies, temperature, turbidity, chlorophyll a, NO₃, SRSi, TN and TP. There was a decreasing trend of the measured parameters towards offshore sites, except for Secchi transparency and NO₃, which increased towards the offshore waters. DO concentrations (mean ± SD) varied between 6.97 ± 0.57 mg O₂ l^–1 and 5.80 ± 0.72 mg O₂ l^–1 in the inshore and offshore waters, respectively. Turbidity values were comparatively higher in the inshore (3.73 ± 2.21 NTU) than the offshore waters (2.19 ± 1.81 NTU). Chlorophyll a concentrations varied between 17.36 ± 6.13 µg l^–1 and 8.09 ± 4.38 µg l^–1 in the inshore and offshore waters, respectively. Increases of unsustainable human activities in the lake and its catchments, plus increased degradation of wetlands, are among the causes of the observed water quality changes. In order to be fruitful and sustainable, the management of Lake Victoria and its catchment needs to take an ecosystem approach, and to involve all key stakeholders.     

Abundance,Species Composition and Spatial Distribution of Planktonic Rotifers and Crustaceans in Lake Ziway (Rift Valley,Ethiopia)

Species composition, abundance, and spatial distribution of rotifer and crustacean zooplankton were studied in Lake Ziway from late April to early July 2004. A total of 49 rotifer species was recorded, with Anuraeopsis fissa, Brachionus angularis, Filinia novaezealandiae, and Trichocerca ruttneri being numerically dominant. Variation in abundance was extremely high, ranging from 2 to 1000+ individuals per litre. There was no significant difference in the distribution of rotifer species between inshore and offshore regions. Crustacean species richness was low, with only five cladoceran and three copepod species occurring in the open water. Moina micrura and Diaphanosoma excisum dominated the cladoceran community, whereas Thermocyclops decipiens was the dominant copepod. Although numerically dominant (75%), rotifers accounted for less than 30% of mean total zooplankton biomass. Peak abundance of crustaceans was observed in May and June, following the onset of the rainy season and increased phytoplankton production. Variation in the spatial distribution of crustacean species was neither observed horizontally between inshore and offshore areas nor vertically in the highly turbid and wind exposed deeper part of the lake. On the other hand, Moina micrura varied significantly in size between inshore and offshore areas. Adult M. micrura dominated offshore, whereas juveniles were more abundant inshore, suggesting a predominantly littoral selective predation on large and adult crustaceans by fish. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Juvenile Anadromous Atlantic Salmon of Three Lakes of Newfoundland

Anadromous Atlantic salmon parr (Salmo salar) were captured from three Newfoundland lakes. Yearling and 2 year parr were captured most often in shallow water <2 m deep around lake perimeters and were not abundant at greater depths. The 3+ and 4+ parr age groups were rarely found inshore (<2 m) but were captured in deeper offshore areas. Though the parr population in one deep lake was too small to be estimated by mark-recapture, estimates of the yearlings plus 2+ age groups in two shallower lakes were 55 and 63 parr ha⁻¹. respectively. No under yearlings were found in lake habitats. Parr density in the three study lakes varied inversely with mean lake depth. Lacustrine parr growth rate was greater than or equal to that of stream dwelling parr. It is concluded that some shallow lakes of Newfoundland provide major rearing areas for juvenile anadromous Atlantic salmon.     

The composition and abundance of phytoplankton in Lake Bukoni,western Uganda

Lake Bukoni is one of the crater lakes in western Uganda. Investigations into this lake is limited compared to other African lakes. Data on phytoplankton ecology in the lake are lacking. Phytoplankton consists of a community of photosynthetic, microscopic plants adapted to suspension in water. They constitute ‘hidden flora’ which make an important contribution to the primary productivity of a water mass. Some phytoplankton taxa, among them species belonging to Cyanophyta, are known to influence ecological transformations and to cause health hazards in water bodies that are used by humans. From July 2004 to December 2005, phytoplankton was collected from two sites (inshore and offshore) in Lake Bukoni. An inverted microscope, Sedgwick counting chamber and multiple tally denominator were used to quantify the phytoplankton. Phytoplankton was dominated by nonheterocystous cyanoprokaryotes especially Lyllgbya limnetica followed by the diatoms Synedra ulna and Fragillaria mutabilis. The inshore site had more phytoplankton species. Differences in phytoplankton diversity and density were mainly attributed to mixing and presence of macrophytes. The occurrence of large numbers of cyanoprokaryotes poses a potential health hazard to the local people who utilize the water from Lake Bukoni. The dominance of cyanoprokaryotes might result in ecological transformations like loss of biodiversity.     

Spatial patterns of seston concentration and biochemical composition between nearshore and offshore waters of a Great Lake

1. The concentration and composition of seston were measured in the east basin of Lake Erie (U.S.A.–Canada) to determine whether the classical patterns of nearshore versus offshore zonation considered typical of large lakes have been altered by decades of change in nutrient loading and food web structure, and to test the possibility that food quality for seston consumers, as reflected in nutrient and lipid composition, may currently be diminished. Chlorophyll‐a (chl‐a), particulate organic carbon (POC), particulate P, total lipid (TL), lipid classes, fatty acids and major phytoplankton groups were compared between the nearshore (<20 m) and offshore zones in spring and early summer. 2. Particulate organic carbon, chl‐a and TL concentrations were lower nearshore than offshore, which is a reversal of the classical large lake pattern but consistent with other evidence from the Great Lakes that colonization by filter‐feeding dreissenid mussels can cause seston depletion in relatively shallow waters. 3. Terrestrial and bacterial fatty acid markers were slightly higher in the nearshore than the offshore, but there was little difference in nutrient, taxonomic or biochemical composition between zones otherwise. This shows a small differential influence of terrestrial and detrital inputs, and a slight shift away from phytoplankton dominance of seston composition in the nearshore, but little difference in potential food quality of seston. 4. Microscope counts, phytoplankton fluorescence, and fatty acid composition indicated succession from diatom dominance in May to dominance by cryptophyte and chromophyte flagellates in June and July. Depletion of dissolved P and Si, increased contribution of storage lipids, and changes in fatty acid composition suggested a mild degree of nutrient deficiency during this spring to summer transition. 5. Total lipids were dominated by classes typical of pigments and membranes with only moderate amounts of neutral (storage) lipids, polyunsaturated fatty acids (PUFA) were abundant (especially the long chain ω‐3 acids) and the POC : chl‐a and POC : particulate P ratios indicated P sufficiency or slight P‐deficiency. Despite concerns that some important consumers in the Great Lakes food web may be suffering from diminished food quality, these results point to a generally high seston food quality even with highly abundant mussels and incipient P deficiency in the east basin of Lake Erie.     

Distribution and breeding biology of offshore cichlids in Lake Malawi/ Niassa

Synopsis Lake Malawi/Niassa is the second largest rift valley lake in Africa, with an area of 28 800 km², and an average and maximum depth of 292 m and>700 m, respectively. The lake is well known for the great diversity of fish occurring in the inshore zone. However, the offshore fish community is poorly documented. To rectify this, regular sampling was undertaken over two years, using trawl and gillnets at six offshore locations. This paper reports on the species composition, spatial distribution and breeding biology of the dominant cichlids species from the offshore pelagic zone. Cichlids formed approximately 88% of the offshore fish biomass. Most abundant were two species of zooplanktivores in the genus Diplotaxodon that made up 71% of the offshore fish biomass. An undescribed species, given the cheironym D. bigeye, was mainly found at a depth of 220 m during the day, but moved into near surface waters at night when the moon was full. This species was absent from the shallow regions of the lake. The most abundant offshore species was D. limnothrissa, which was distributed evenly throughout the lake to depths of 220 m. A less common offshore zooplanktivore was Copadichromis quadrimaculatus that formed 5% of the biomass and was confined to the upper 100 m of the water column. The main piscivores were in the genus Rhamphochromis and formed approximately 10% of the offshore fish biomass. The two dominant taxa were R. longiceps and the large Rhamphochromis group, and both were more common in the southern half of the lake. The former occurred mainly in the upper 100 m of the water column and the latter mainly at depths of 100–150 m. The length at maturity and fecundity for the dominant offshore species were estimated and seasonal breeding cycles determined from gonad activity and gonado-somatic indices.     

Distribution of sawfishes (Pristidae) in the Queensland Gulf of Carpentaria,Australia, with notes on sawfish ecology

Synopsis Pristis microdon, P. zijsron, P. clavata and Anoxypristis cuspidata are distributed throughout the Queensland section of the Gulf of Carpentaria, Australia. In a survey of the four species, Anoxypristis cuspidata was the most abundant and was recorded in both the inshore and offshore set net fisheries. The size distribution and catch locations of A. cuspidata suggest that the inshore area to a depth of 10 m may be the preferred habitat for juveniles of this species, while adults primarily occur offshore. Pristis microdon, P. zijsron and P. clavata were recorded only in the inshore fishery with catches dominated by immature animals. Pristis microdon was caught in the inshore fishery late in the monsoonal wet season (February to April) and inhabited both freshwater and estuarine environments. Pristis zijsron occurred only on the sand and mud flats outside river mouths whilst P. clavata inhabited both the sand and mud flats and upstream estuarine habitats. Observations on reproductive staging and the capture of neonate specimens suggest that in all four pristids, pupping occurred through the wet season until the beginning of the dry season in May. A seasonal set net closure for the barramundi, Lates calcarifer and shark fisheries, which has been in place since 1980 in Queensland Gulf waters, therefore offers a measure of protection to breeding female sawfish and their offspring.     

Increased nutrient loading and rapid changes in phytoplankton expected with climate change in stratified South European lakes: sensitivity of lakes with different trophic state and catchment properties

We hypothesised that increasing winter affluence and summer temperatures, anticipated in southern Europe with climate change, will deteriorate the ecological status of lakes, especially in those with shorter retention time. We tested these hypotheses analysing weekly phytoplankton and chemistry data collected over 2 years of contrasting weather from two adjacent stratified lakes in North Italy, differing from each other by trophic state and water retention time. Dissolved oxygen concentrations were higher in colder hypolimnia of both lakes in the second year following the cold winter, despite the second summer was warmer and the lakes more strongly stratified. Higher loading during the rainy winter and spring increased nutrient (N, P, Si) concentrations, and a phytoplankton based trophic state index, whilst the N/P ratio decreased in both lakes. The weakened Si limitation in the second year enabled an increase of diatom biovolumes in spring in both lakes. Chlorophyll a concentration increased in the oligo-mesotrophic lake, but dropped markedly in the eutrophic lake where the series of commonly occurring cyanobacteria blooms was interrupted. The projected increase of winter precipitation in southern Europe is likely to increase the nutrient loadings to lakes and contribute to their eutrophication. The impact is proportional to the runoff/in-lake concentration ratio of nutrients rather than to the retention time, and is more pronounced in lakes with lower trophy.     

A seasonal study of the distributions of surface state variables in Liverpool Bay. III. An offshore front

Variously orientated transects through Liverpool Bay in December, 1976, revealed a narrow but steep density discontinuity running north to south through the surface waters. Surface distribution maps from cruises during the winter of 1976–1977 showed that the density discontinuity provided a sharp line of demarcation between the physical and chemical characteristics of the offshore and coastal waters. Over the winter, an accumulation of river discharged nutrients in the waters bounded by the front and the coast generated a large nutrient reservoir in the inshore waters and propagated a difference in the nutrient availability ratios across the front. As a consequence, by March the inshore waters were characterized by lower salinities and temperatures, higher dissolved inorganic nutrient concentrations and lower silicon to total nitrogen ratios than the offshore waters. Prior to the 1977 spring phytoplankton bloom in Liverpool Bay, these two contrasting water types were in juxtaposition along the length of the front.     

Phytoplankton ecology of the Lake of Menteith,Scotland

The results discussed in this paper represent the first seasonal ecological study carried out on the phytoplankton of the Lake of Menteith. All measured nutrients reached maximum levels during the winter, with silicate showing particularly high concentrations (up to 85 µg at Si l^–1). During the summer period phosphate, nitrate and silicate showed almost complete exhaustion in surface waters. The lake water was consistently alkaline, never falling below pH 7, while the alkalinity ranged from 20 to 24 mg CaCO₃ l^–1. Generally, the nutrient status of the main inflow had a rapid effect on the water quality of the lake.The region of the lake under investigation showed no thermal stratification at any period of the year, although continuous thermal gradients were recorded in the winter. The continual circulation of the water mass probably prevented oxygen saturation from falling below 77% even following a large phytoplankton bloom and subsequent decomposition.From an examination of net phytoplankton samples the Lake of Menteith could be described as blue-green or blue-green/diatom in nature. From the quantitative study, large pulses of Melosira, Asterionella and Fragilaria were recorded in the spring. The disappearance of the species appears to be related to silicate limitation. The summer growth of Asterionella may have been promoted by a nitrogen source other than nitrate and nitrite, both of which were reduced to critical levels. This alternative source of combined nitrogen may have been contributed by nitrogen-fixing algae in the lake. Three species of Anabaena were recorded, all of which produced large populations during the year.Department of Botany, The University of GlasgowPresent Address: Department of Biology, College of Science, University of Sulaimaniyah, Sulaimaniyah, Iraq     

Resource allocation in the submerged plant Vallisneria natans related to sediment type, rather than water-column nutrients

1. Phenotypic plasticity in resource allocation by Vallisneria natans was investigated in a greenhouse experiment, using three types of sediment [sandy loam, clay, and a 50 : 50 (by volume) mixture of the two sediments] and two levels of water‐column nutrient. The clay was collected from a highly eutrophic lake in Jiangsu Province, China, and the N and P concentrations applied in nutrient media were at the upper limits observed in most lakes of China. 2. Growth and biomass allocation were significantly affected by sediment type, rather than water‐column nutrients. Plant growth in clay and the mixture were similar, and 2.4–3.4 times higher than that in sandy loam. Compared with the plants grown in clay or the mixed sediments, the plants grown in sandy loam allocated relatively more biomass to root (11–17% versus 7–8% of total biomass), and relatively less to leaf (76–82% versus 86–87% of total biomass). Plastic variations in root area were induced by sediment type alone (P < 0.05), whereas the impacts of sediment type and water‐column nutrients on leaf area were insignificant (P > 0.05). 3. Plant N and P concentrations were significantly affected by both sediment type and water‐column nutrients. Increased nutrient availability in the water column enhanced plant N concentration by 3.5–20.2%, and plant P concentration by 19.1–25.8%. 4. Biomass accumulation and plant nutrient concentration in plants grown in different sediment types and water‐column nutrients indicate that sediment type had more significant impacts on growth and N and P concentrations of V. natans than did water‐column nutrients. Changes in phenotype are a functional response to nutrient availability in sediment, rather than to water‐column nutrients.     

Colony morphology,age structure,and relative growth of two gorgonian corals,Leptogorgia hebes (Verrill) and Leptogorgia virgulata (Lamarck), from the northerm Gulf of Mexico

Along the northeastern Gulf of Mexico coast, Leptogorgia hebes is the predominant gorgonian at two inner shelf (22 and 27 m) sites, while Leptogorgia virgulata dominates a third shallow (1–2m), inshore site. The axial skeleton of both species is composed of concentric rings which appeared to exhibit annual periodicity. Although age structure of the populations differed between shelf sites, neither colony growth rates, estimated from direct measurements of ring widths plotted on Walford graphs, nor overall branching complexity, determined from bifurcation ratios (R_b), differed between the shelf populations. Larger individuals at the 27m shelf site were simply older than those at the shallower (22m) shelf site. Walford plots of growth increments revealed colony growth of the shallow inshore species to be similar to that of the offshore species.     

Carbon,phosphorus and nitrogen budgets of the littoral Equisetum belt in an oligotrophic lake

Stores and flows of carbon, phosphorus and nitrogen in a littoral Equisetum stand were studied in 1978–1980 in the oligotrophic, mesohumic lake Pääjärvi, southern Finland. The major carbon and nutrient stores were sediment and Equisetum. The seasonal cycle of the macrophyte vegetation had a profound influence on the whole littoral ecosystem. In spring, when only dead remains of Equisetum were present above ground, there were few differences in nutrient, chlorophyll a and zooplankton concentrations between the littoral and the open lake; phytoplankton and epiphytes were the major producers.In early June, when new shoots of Equisetum reached the water surface, water exchange between the littoral and the open lake started to diminish, and the characteristic features of a closed macrophyte zone gradually developed: by August the P, Chl a and zooplankton concentrations in the littoral were 5–10 times those in the open lake. From late June until autumn Equisetum was overwhelmingly dominant both in biomass and in production.The measured total primary production and respiration values indicated a high rate of internal cycling of carbon and nutrients. The daily P requirements of plant growth exceeded the total P stored in the water by a factor of 2–4, and also exceeded the release of nutrients in excretion. High N:P ratios in the water (total 10–64, inorganic 18–171) suggested that P was probably always the limiting nutrient.The P content of the annual production of Equisetum in Pääjärvi was 2.3% of the mean annual P load, and 5.3% of the mean total P storage in the water volume of the lake.     

Microcystin production by cyanobacteria in the Mwanza Gulf (Lake Victoria,Tanzania)

In order to investigate the potential for microcystin (MC) production by cyanobacteria in the Mwanza Gulf (Lake Victoria, Tanzania), nutrients, phytoplankton and microcystins were sampled inshore (3 m depth) and offshore (18 m depth) from May to August 2002. Significant differences in soluble reactive phosphorus (SRP) and nitrate concentrations between offshore and inshore indicated eutrophication via terrestrial run-off. Though the concentrations of SRP and nitrate ranged between 36–127 and 35–726 μg l ⁻¹ each, the phytoplankton biovolume was generally low. The phytoplankton community was dominated by diatoms (Nitzschia acicularis), a number of cyanobacterial species (Aphanocapsa sp., Anabaena sp., Planktolyngbya spp., Microcystis sp.) and cryptomonads. The water column was completely mixed and Nitzschiapeaked in abundance during July. All cyanobacteria were low in abundance during the entire study period (0.1–1.6 mm ³ l ⁻¹). Microcystins were analysed using high performance liquid chromatography coupled with diode array detection High Performance Liquid Chromatography with Diode Array Detection (HPLC-DAD) and in most samples no microcystins were detected. The highest concentration of [Asp ³]-MC-RR was found in open water at the surface on July 2nd, 2002 (1 μg l ⁻¹). MC concentrations did not pose a potential health risk in the Mwanza Gulf during the study period, however, it is possible that the period of higher cyanobacterial biovolumes has been missed during the sampling period of this study.     

Seasonal changes in the chemistry and biology of a meromictic lake (Big Soda Lake,Nevada, U.S.A.)

Big Soda Lake is an alkaline, saline lake with a permanent chemocline at 34.5 m and a mixolimnion that undergoes seasonal changes in temperature structure. During the period of thermal stratification, from summer through fall, the epilimnion has low concentrations of dissolved inorganic nutrients (N, Si) and CH₄, and low biomass of phytoplankton (chlorophyll a ca. 1 mgm ^-3). Dissolved oxygen disappears near the compensation depth for algal photosynthesis (ca. 20 m). Surface water is transparent so that light is present in the anoxic hypolimnion, and a dense plate of purple sulfur photosynthetic bacteria (Ectothiorhodospira vacuolata) is present just below 20 m (Bchl a ca. 200 mgm^-3). Concentrations of N H₄ ⁺, Si, and CH₄ are higher in the hypolimnion than in the epilimnion. As the mixolimnion becomes isothermal in winter, oxygen is mixed down to 28 m. Nutrients (NH₄ ⁺, Si) and CH₄ are released from the hypolimnion and mix to the surface, and a diatom bloom develops in the upper 20 m (chlorophyll a > 40 mgm^-3). The deeper mixing of oxygen and enhanced light attenuation by phytoplankton uncouple the anoxic zone and photic zone, and the plate of photosynthetic bacteria disappears (Bchl a ca.10mgm^-3). Hence, seasonal changes in temperature distribution and mixing create conditions such that the primary producer community is alternately dominated by phytoplankton and photosynthetic bacteria: the phytoplankton may be nutrient-limited during periods of stratification and the photosynthetic bacteria are light-limited during periods of mixing.     

Algal responses to experimental nutrient addition in the littoral community of a subtropical lake

1. An in situ experiment was performed in the littoral zone of a large, subtropical lake to quantify effects of phosphorus (P) and nitrogen (N) on algal biomass, productivity, nutrient content and phosphate uptake kinetics. 2. We hypothesized that resident periphyton rapidly sequester added nutrients from the water column, but once a certain threshold is reached, nutrients remain in the water and permit a shift to a phytoplankton-dominated community. 3. Three duplicate sets of 1.2-m diameter mesocosms were treated with 10, 20 or 50 μg P L^??1 in combination with 100, 200 or 500 μg N L^??1, respectively. The nutrients were added thrice weekly for 14 days, after which the treatment doses were doubled for an additional 9 days. The cumulative amounts of P and N added over the course of the study were 700 and 7000 μg L^??1, respectively. Two untreated mesocosms and two open reference sites were used as controls. 4. The total P concentration in the water column of nutrient-treated mesocosms remained low, even after prolonged high dosing. However, there was a two-fold increase in the P content of surface algal mats and epiphyton. This indicates that some of the added P was sequestered by those components of the community. In contrast, metaphyton and epipelon displayed little or no increase in their P content. Large quantities of added P could not be accounted for in the periphyton community, and may reflect unmeasured losses to the sediments or other pools. 5. Nitrogen also was depleted from the water column, but there were no significant increases in periphyton N content. Much of the added N could not be accounted for in mass balances, and may have been lost from the mesocosms through volatilization or other biochemical processes. 6. Chlorophyll-a in epiphyton increased significantly after 14 days in the highest nutrient treatment, where there also was a proliferation of Spirogyra on day 28. 7. On day 28, water column samples from the highest nutrient treatment also displayed a significantly higher rate of carbon uptake, and a significantly higher concentration of midday dissolved oxygen. 8. The hypothesis that phytoplankton become dominant at high nutrient loading rates was not supported. However, there were dramatic changes in community structure (increased dominance by epiphytic Spirogyra) and function (increased productivity and dissolved oxygen) in response to nutrient additions.