Shifting limitation of primary production: experimental support for a new model in lake ecosystems

Effects of water depth on the biomass of two dominant submerged macrophyte species in floodplain lakes during flood and dry seasons

Floodplain lakes share characteristics of both deep and shallow lakes throughout any given year. Seasonal fluctuations in their water depth drive changes in nutrients and total primary productivity, which directly and indirectly affect submerged macrophyte biomass. To investigate how water depth and environmental variables affect submerged macrophyte biomass, we surveyed six sub-lakes in the Poyang Lake floodplain, China, during the flood and dry seasons of 2021. Dominant submerged macrophytes include Vallisneria spinulosa and Hydrilla verticillata. The effect of water depth on the biomass of these macrophytes varied between the flood and dry seasons. In the flood season, there was a direct effect of water depth on biomass, while in the dry season only an indirect effect was observed. During the flood season, the direct effect of water depth on the biomass of V. spinulosa was less than the indirect effect, with water depth primarily affecting the total nitrogen, total phosphorus and water column transparency. Water depth directly, positively affected H. verticillata biomass, with this effect being greater than the indirect effect by affecting the carbon, nitrogen and phosphorus content in the water column and sediment. During the dry season, water depth affected H. verticillata biomass indirectly through sediment carbon and nitrogen content, while for V. spinulosa, the effect on biomass was indirect through carbon content of the sediment and water column. The main environmental variables affecting submerged macrophyte biomass in the Poyang Lake floodplain during the flood and dry seasons, and the mechanisms through which water depth affects dominant submerged macrophyte biomass, are identified. An understanding of these variables and mechanisms will enable improved management and restoration of wetland.

Climate Warming Does Not Override Eutrophication, but Facilitates Nutrient Release from Sediment and Motivates Eutrophic Process

The climate is changing. The average temperature in Wuhan, China, is forecast to increase by at least 4.5 °C over the next century. Shallow lakes are important components of the biosphere, but they are sensitive to climate change and nutrient pollution. We hypothesized that nutrient concentration is the key determinant of nutrient fluxes at the water-sediment interface, and that increased temperature increases nutrient movement to the water column because warming stimulates shifts in microbial composition and function. Here, twenty-four mesocosms, mimicking shallow lake ecosystems, were used to study the effects of warming by 4.5 °C above ambient temperature at two levels of nutrients relevant to current degrees of lake eutrophication levels. This study lasted for 7 months (April–October) under conditions of near-natural light. Intact sediments from two different trophic lakes (hypertrophic and mesotrophic) were used, separately. Environmental factors and bacterial community compositions of overlying water and sediment were measured at monthly intervals (including nutrient fluxes, chlorophyll a [chl a], water conductivity, pH, sediment characteristics, and sediment-water et al.). In low nutrient treatment, warming significantly increased chl a in the overlying waters and bottom water conductivity, it also drives a shift in microbial functional composition towards more conducive sediment carbon and nitrogen emissions. In addition, summer warming significantly accelerates the release of inorganic nutrients from the sediment, to which microorganisms make an important contribution. In high nutrient treatment, by contrast, the chl a was significantly decreased by warming, and the nutrient fluxes of sediment were significantly enhanced, warming had considerably smaller effects on benthic nutrient fluxes. Our results suggest that the process of eutrophication could be significantly accelerated in current projections of global warming, especially in shallow unstratified clear-water lakes dominated by macrophytes.

Dark matters: Contrasting responses of stream biofilm to browning and loss of riparian shading

Concentrations of terrestrial-derived dissolved organic carbon (DOC) in freshwater ecosystems have increased consistently, causing freshwater browning. The mechanisms behind browning are complex, but in forestry-intensive regions browning is accelerated by land drainage. Forestry actions in streamside riparian forests alter canopy shading, which together with browning is expected to exert a complex and largely unpredictable control over key ecosystem functions. We conducted a stream mesocosm experiment with three levels of browning (ambient vs. moderate vs. high, with 2.7 and 5.5-fold increase, respectively, in absorbance) crossed with two levels of riparian shading (70% light reduction vs. open canopy) to explore the individual and combined effects of browning and loss of shading on the quantity (algal biomass) and nutritional quality (polyunsaturated fatty acid and sterol content) of the periphytic biofilm. We also conducted a field survey of differently colored (4.7 to 26.2 mg DOC L^-1 ) streams to provide a 'reality check' for our experimental findings. Browning reduced greatly the algal biomass, suppressed the availability of essential polyunsaturated fatty acids, especially eicosapentaenoic acid (EPA), and sterols, but increased the availability of terrestrial-derived long-chain saturated fatty acids (LSAFA). In contrast, loss of shading increased primary productivity, which resulted in elevated sterol and EPA contents of the biofilm. The field survey largely repeated the same pattern: biofilm nutritional quality decreased significantly with increasing DOC, as indicated particularly by a decrease of the ω-3:ω-6 ratio and increase in LSAFA content. Algal biomass, in contrast, was mainly controlled by dissolved inorganic nitrogen (DIN) concentration, while DOC concentration was of minor importance. The ongoing browning process is inducing a dramatic reduction in the nutritional quality of the stream biofilm. Such degradation of the major high-quality food source available for stream consumers may reduce the trophic transfer efficiency in stream ecosystems, potentially extending across the stream-forest ecotone.

Resolving the Drivers of Algal Nutrient Limitation from Boreal to Arctic Lakes and Streams

Nutrient inputs to northern freshwaters are changing, potentially altering aquatic ecosystem functioning through effects on primary producers. Yet, while primary producer growth is sensitive to nutrient supply, it is also constrained by a suite of other factors, including light and temperature, which may play varying roles across stream and lake habitats. Here, we use bioassay results from 89 lakes and streams spanning northern boreal to Arctic Sweden to test for differences in nutrient limitation status of algal biomass along gradients in colored dissolved organic carbon (DOC), water temperature, and nutrient concentrations, and to ask whether there are distinct patterns and drivers between habitats. Single nitrogen (N) limitation or primary N-limitation with secondary phosphorus (P) limitation of algal biomass was the most common condition for streams and lakes. Average response to N-addition was a doubling in biomass; however, the degree of limitation was modulated by the distinct physical and chemical conditions in lakes versus streams and across boreal to Arctic regions. Overall, algal responses to N-addition were strongest at sites with low background concentrations of dissolved inorganic N. Low temperatures constrained biomass responses to added nutrients in lakes but had weaker effects on responses in streams. Further, DOC mediated the response of algal biomass to nutrient addition differently among lakes and streams. Stream responses were dampened at higher DOC, whereas lake responses to nutrient addition increased from low to moderate DOC but were depressed at high DOC. Our results suggest that future changes in nutrient availability, particularly N, will exert strong effects on the trophic state of northern freshwaters. However, we highlight important differences in the physical and chemical factors that shape algal responses to nutrient availability in different parts of aquatic networks, which will ultimately affect the integrated response of northern aquatic systems to ongoing environmental changes.

Impacts of nutrient loading and fish grazing on the phytoplankton community and cyanotoxin production in a shallow tropical lake: Results from mesocosm experiments

Given the increasing eutrophication of water bodies in Africa due to increasing anthropogenic pressures, data are needed to better understand the responses of phytoplankton communities to these changes in tropical lakes. These ecosystems are used by local human populations for multiple purposes, including fish and drinking water production, potentially exposing these populations to health threats if, for example, an increase in toxic cyanobacterial blooms is associated with increasing eutrophication. To test the short-term response of the phytoplankton community to the addition of nutrients (phosphorus and nitrogen, alone or in combination) and Nile tilapia, we developed an in situ mesocosm experiment in a freshwater lagoon located near Abidjan (Ivory Coast). We found that phytoplankton growth (estimated by chlorophyll-a quantification) was highly stimulated when both nitrogen and phosphorus were added, while there was no clear evidence for such colimitation by these two nutrients when considering their concentrations in the lagoon. Phytoplankton growth was accompanied by significant changes in the diversity and composition of this community and did not lead to an increase in the proportions of cyanobacteria. However, the addition of fish to some mesocosms resulted in a drastic decrease in phytoplankton biomass and a dominance of chlorophytes in this community. Finally, these experiments showed that the addition of nitrogen, alone or combined with phosphorus, stimulated microcystin production by cyanobacteria. In addition, no evidence of microcystin accumulation in the fish was found. Taken together, these data allow us to discuss strategies for controlling cyanobacterial blooms in this tropical ecosystem.

Driving mechanisms of gross primary productivity geographical patterns for Qinghai-Tibet Plateau lake systems

Being a fundamental property of aquatic systems, gross primary productivity (GPP) is affected by complex environmental factors, such as salinity, nutrients, pH, and sunlight. Under conditions of intensified anthropogenic activity and climate change, it is critical to understand the driving mechanisms of GPP in alpine lakes. In this study, we investigated GPP and associated environmental factors of 23 lake systems in the Qinghai-Tibet Plateau (QTP) along an altitudinal range (from 2500 m to 4500 m). Results showed an increase in chlorophyll a (Chl a) content as altitude increased and a corresponding decrease as salinity increased. Furthermore, geographical patterns of GPP were higher at the mid-gradient and lower at the extreme gradient. Higher solar radiation and water temperatures, stronger evaporation and higher salinity levels, and lower pH and higher nutrient content were all driving mechanisms of GPP in low altitudinal lake systems within high latitudinal regions. Such conditions have collectively resulted in the current GPP pattern via the promotion or inhibition of phytoplankton growth and photosynthesis. Specifically, geographical features and climate change jointly drive algal growth and GPP of alpine lake systems via internal circulation processes; however, anthropogenic activities interfere with external circulation processes for most of lower-middle altitudinal lake systems, thus playing a certain role in regulating environmental factors and GPP alongside climate change.

Diet mixing: do animals integrate growth or resources across temporal heterogeneity?

Animals commonly experience spatial and temporal variation in resource quality, thus experiencing temporally variable diets. Methods for scaling up growth in component patches to long‐term growth across heterogeneity are seldom explicitly considered. Long‐term growth is sometimes considered to be a weighted average of growth rates on component diets (growth integration). However, if animals integrate resources across high‐ and low‐quality diets, their long‐term growth may be greater than predicted from diet‐specific growth rates (resource integration). We measured biomass growth rates of seven Daphnia species exposed to different types of diet variation in algal phosphorus (P) content. Support for resource integration was found for four of the seven species, which achieved near maximal growth when high‐P food was available for at least 12 h. In contrast, no support for resource integration was found for the other three species. These three species achieved only one‐half maximal growth rate under the same conditions and could be considered growth integrators. The type of integration could be predicted from the degree of stoichiometric homeostasis. Species with weak homeostatic regulation exhibited a capacity for resource integration. Resource integrators should have an advantage in heterogeneous environments.