The role of planktonic ciliates in lake ecosystems

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The production and fate of picoplankton and protozoa in the pelagic food web of Napoleon Gulf, Lake Victoria, East Africa

The importance of the microbial food web and how it interplays with the classical food chain has gained considerable attention in temperate lakes. However its role in carbon transfer from pico- and nanoplankton to zooplankton and planktivores is relatively unknown in tropical lakes. Sampling of the microbial food web and experiments to estimate the growth rate and fate of its components were performed in Lake Victoria, East Africa, during the mixing season (May to August) 2002. Bacterioplankton and ciliate densities in Napoleon Gulf ranged from 6. 2 to 14. 9 cells x 106•mL-1 and 51. 9 to 75. 2 cells•mL-1, respectively. Flagellate abundance was high, ranging from 70. 4 to 127. 9 cells x 103•mL-1. Small flagellates, tentatively called Choanoflagellida, dominated the flagellate community by abundance and biomass. Bacterial growth rates were low, yet high abundance and cell size resulted in high bacterial production representing 24 to 38% of phytoplankton production. Protozoan growth rates and production are similar to values reported for other African lakes and the Laurentian Great Lakes. Protozoa were the dominant grazers of bacteria with grazing pressure switching from protozoa > 5 µm in June to protozoa 40-µm plankton. Given that plankton of Lake Victoria is dominated by colonial cyanobacteria and raptorial zooplankton, protozoa could be an important pathway in the pelagic food web of Lake Victoria, East Africa

Carbon Dynamics in Tropical Lake Malawi

Large lakes of the world play a vital role in the global carbon cycle as they act both as conduits and sinks of terrestrially and atmospherically derived carbon. Lake Malawi, lying at the extreme southernmost end of the East African Rift Valley is one of the largest, deepest and most ancient of the African Great Lakes. In this study, the spatial and seasonal variation of direct measurements of air and water pCO2 were taken for a period of one annual cycle using a vessel of opportunity along the north-south axis of Lake Malawi. These data, together with limnological and meteorological variables, were used to estimate the annual net CO2 flux at the air-water interface. The data reveal distinct spatial and temporal variation in pCO2 and CO2 flux that is related to hydrodynamic and meteorological conditions that drive nutrient dynamics and phytoplankton productivity. Contemporaneous measurements of lake temperature profiles, nutrients, weather conditions, phytoplankton biomass and seston δ13C suggest that increased nutrient supply due to vertical mixing and allochthonous inputs promotes high phytoplankton growth rates and CO2 uptake during the cool, mixing season and the hot, rainy season. Spatially, the southernmost region of the lake which is the most nutrient-rich and hence most productive was distinct from the rest of the lake. High CO2 efflux to the atmosphere was observed in this region at the onset of the cool, mixing season probably due to the physical resupply of dissolved inorganic carbon (DIC) from deep waters during upwelling. Seasonally, almost the entire lake was CO2 undersaturated with respect to the atmosphere during the wet, hot season (December to April) and the cool, mixing season (July to September), periods when nutrient supply from river inputs and vertical mixing that promote phytoplankton photosynthesis are high. By contrast, during the hot, stratified season (October and November), CO2 evasion to the atmosphere was observed, possibly driven by high respiration to photosynthesis ratios. The experiments conducted to determine the influence of river water loading and vertical exchange on the metabolism of Lake Malawi using Linthipe River water and hypolimnetic water from Lake Malawi shows distinct differences. River loading results in CO2 supersaturation implying high respiration rates while hypolimnetic water showed a net consumption of carbon dioxide. Low phytoplankton biomass and particulate organic carbon production were observed in incubation bottles spiked with river water. In contrast, bottles spiked with hypolimnetic water showed high phytoplankton biomass and particulate organic carbon. The high OC: DP ratio compared to lake seston stoichiometry in Linthipe River is responsible for the observed heterotrophy while autotrophy by hypolimnetic water was sustained by the relatively low OC: DP from vertical flux. Autochthonous primary production constitutes the major source of organic carbon in the lake and although concentrations of DOC and POC are relatively low compared to other lakes, the internal organic carbon inventory is large. The vertical exchange is an important source of DIC to the upper 200 m of the lake and it appears the recycling rate of carbon decreases with depth. A comparison of carbon sedimentation rates and DIC vertical flux rates among different strata in the lake suggests that the carbon recycling efficiency within the epilimnion is 73%, while it is 33% within the anoxic hypolimnion. If carbon is selectively retained while P is efficiently recycled into the epilimnion, CO2 fixation in the epilimnion will be enhanced leading to autotrophy. Several studies have indicated that the majority of oligotrophic inland waters are net sources of CO2 to the atmosphere. Results from the present study indicate that this paradigm may not apply to large tropical lakes. On an annual basis, Lake Malawi is a net CO2 sink and hence net autotrophic, absorbing 209 to 320 mmol C/m/yr from the atmosphere. Using the Lake Malawi C:P stoichiometry requirements and the carbon mass balance approach, we still determined that the lake is a net CO2 sink. The data further suggest that surface pCO2 variability is driven primarily by biological processes and vertical mixing, with seasonal temperature fluctuations playing a minor role. The variability in CO2 underscores the importance of making measurements with high spatial and temporal resolution to accurately determine air-water gas fluxes in large lakes

THE BIOACCUMULATION OF CYANOTOXINS IN AQUATIC FOOD WEBS

Cyanobacteria are naturally occurring photosynthetic bacteria, ubiquitous in nature. Increases in temperature and nutrients have supported the proliferation of cyanobacterial growth globally, especially in freshwater systems. Many taxa can produce biotoxins referred to as “cyanotoxins”. While toxic cyanobacteria are a growing public health concern, little is known about the accumulation of cyanotoxins in lake food webs. This research investigates the seasonal occurrence and the potential role of toxic cyanobacteria in two lakes of contrasting water qualities and food web structures. Objectives of this study were to test the bioaccumulation of microcystins (MCs) and beta-methyl-alanine-amino acid (BMAA) in zooplankton. I further assessed the major zooplankton and phytoplankton communities using stable isotopes of carbon and nitrogen, with focus on the importance of zooplankton consumer types and diet sizes; picoplankton, nanoplankton and net plankton. The bioaccumulation of cyanotoxins in zooplankton were dependent on the trophic levels and feeding behaviors of the zooplankton, which vary by species, seasons and lakes. Cyanotoxin transfer was also dependent on the presence and composition of toxic cyanobacteria, including picocyanobacteria. Understanding the transfer of cyanotoxins to the zooplankton community have significant implications in determining the pathways and the bioaccumulation of cyanotoxins to higher trophic levels such as fish, wildlife and humans

Dissolved organic matter composition and reactivity in Lake Victoria, the World’s largest tropical lake

peer reviewedWe report a data set of dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) composition (stable carbon isotope signatures, absorption and fluorescence properties) obtained from samples collected in Lake Victoria, a large lake in East Africa. Samples were collected in 2018-2019 along a bathymetric gradient (bays to open waters), during three contrasting seasons: long rainy, short rainy and dry, which corresponded to distinctly water column mixing regimes, respectively, stratified, semi-stratified and mixed regimes. Eight DOM components from parallel factor analysis (PARAFAC) were identified based on three-dimensional excitation–emission matrices (EEMs), which were aggregated into three main groups of components (microbial humic-like, terrestrial humic-like, protein-like). Spatially, the more productive bays were characterized by higher DOM concentration than deeper more offshore waters (fluorescence intensity and DOC were ~80% and ~30% higher in bays, respectively). Seasonally, the DOM pool shifted from protein-like components during the mixed regime to microbial humic-like components during the semi-stratified regime and to terrestrial humic-like components during the stratified regime. This indicates that pulses of autochthonous DOM derived from phytoplankton occurred when the lake was mixing, which increased the availability of dissolved inorganic nutrients. Subsequently, this freshly produced autochthonous DOM was microbially processed during the following semi-stratified regime. In the open waters, during the stratified regime, only terrestrial refractory DOM components remained because the labile and fresh stock of DOM created during the preceding mixed season was consumed. In the bays, the high terrestrial refractory DOM during the stratified regime may be additionally due to the allochthonous DOM input from the runoff. At the scale of the whole lake, the background refractory DOM probably comes mainly from precipitation and followed by river inputs.LAVIGA

River Influence on the Nearshore Ecosystem of Western Lake Superior

As the interface between the terrestrial landscape and the open lake, nearshore areas of the Great Lakes play an important role in modulating whole-lake response to inputs of nutrients and energy from the watershed. These inputs occur primarily via tributary loading, and so it is critical to understand the dynamics of river plumes and the fate of organic carbon and nutrients delivered in the plumes. To assess the influence of river plumes on the biogeochemistry and metabolism of the Lake Superior nearshore zone, the spatial and temporal distribution of turbidity, nutrients, phytoplankton, dissolved oxygen, and dissolved carbon dioxide were measured in the western arm of Lake Superior and select tributaries from June 2016 through October 2016. This study focused on the nearshore ecosystem response to a large storm even in July, showing how the nearshore zone transitioned from a highly turbid, low productivity system immediately following the storm to high phytoplankton productivity after a one-month lag. A steady decrease in surface pCO2 in the month following suggests that increased water clarity and nutrient concentration following the plume event drive nearshore primary production. The shift towards net heterotrophy immediately following the storm event appears to be more so due to decreased water clarity and associated suppressed phytoplankton primary production rather than increased biological breakdown of dissolved organic carbon

Nutrient function over form: Organic and inorganic nitrogen additions have similar effects on lake phytoplankton nutrient limitation

The concentration of dissolved organic nitrogen (DON) is increasing in many northern hemisphere lakes, yet its use by phytoplankton and fate in the environment seldom have been quantified. We conducted 1 week, insitu, microcosm incubations across 25 lakes in northeastern North America to understand how DON, dissolved norganic nitrogen (DIN), and dissolved inorganic phosphorus (P) affected phytoplankton biomass. In addition,we tested whether lakes were limited by single macronutrients (N or P) or colimited by both. Phytoplankton biomass in 80% of lakes responded similarly to DON and DIN additions. Of the lakes where N form produced differential responses, the majority of phytoplankton communities exhibited greater biomass accumulation with DON than DIN. Colimitation was the most common type of nutrient limitation among the study lakes,followed by P limitation. Limitation type shifted with N form in 40% of the study lakes, but without consistent patterns explaining how shifts occurred. Regardless of N form, lakes with watersheds more dominated by agriculture and higher total dissolved nitrogen (TDN) tended to show P‐limited phytoplankton responses, while lakes with less agricultural watersheds and lower TDN tended to show colimited phytoplankton responses.Finally, ambient TDN and total phosphorus (TP) nutrient concentrations were stronger predictors of limitation type than ambient TDN : TP ratios. The different contributions of DON and DIN to phytoplankton biomass insome of our study lakes suggest that DON loading from surrounding watersheds may be an overlooked compo-nent in predicting phytoplankton productivity and nutrient limitation dynamics in freshwater ecosystems

Plankton Communities

Plankton is a group of small organisms that are passively displaced by water, that is, they are dragged by marine tides and currents. Marine plankton, which includes organisms such as protozoa, microalgae, small crustaceans, and jellyfish, play an important role in maintaining the health and balance of the ocean and its complex food chains. Over three sections and eight chapters, this book provides a comprehensive overview of zooplankton and phytoplankton as well as their environmental and economic importance