Phytoplankton In Shallow Lakes Research Articles

Why do algal blooms intensify under reduced nitrogen and fluctuating phosphorus conditions: The underappreciated role of non‐algal light attenuation

AbstractLight availability for phytoplankton in shallow lakes is closely related to non‐algal light attenuation (KdNALA, the fractional light absorption by non‐algal substances, such as suspended sediments); thus, significant changes in global wind speed in recent decades may have a profound effect on light availability and algal blooms in shallow lakes. Herein, the eutrophic shallow Lake Taihu was selected to investigate the long‐term dynamics of light availability and its effect on algal blooms. The results showed that KdNALA decreased from approximately 3.5 to 2.5 m−1 with decreasing wind speed from 2005 to 2021, indicating the significantly increased light availability for phytoplankton despite surface photosynthetically active radiation exhibiting limited variability during the study period. In addition, both experimental (i.e., in situ nutrient enrichment experiments) and statistical approaches (i.e., deviations of Trophic State Index subindices) indicated that phytoplankton growth was primarily light‐limited during the study period in Lake Taihu. Consequently, considerable increases in fraction of observed and maximal chlorophyll a yield at given nitrogen or phosphorus concentration were observed and were mostly related to decreasing KdNALA, which implied that decreasing KdNALA allowed phytoplankton utilize “unused nutrient‐capacity” until the additional algal‐turbidity induce further light limitation or nutrient limitation. As the effect of changes in global wind speed on KdNALA and algal growth received limited attention in the existing research, we revealed an underappreciated mechanism by which global changes in wind speed significantly affects algal biomass by influencing light availability, which may have profound effects on future algal bloom mitigation efforts in shallow lakes.

The changing characteristics of phytoplankton community and biomass in subtropical shallow lakes: Coupling effects of land use patterns and lake morphology

The community composition and biomass of phytoplankton in shallow lakes are impacted by many environmental factors including water quality physicochemical parameters, land use in the watershed, and lake morphology. However, few studies have simultaneously evaluated the relative importance of these factors on the effect of community composition and biomass of phytoplankton. The relative importance of the water quality physicochemical parameters (water temperature [WT], total nitrogen [TN], total phosphorus [TP], pH, dissolved oxygen [DO], electrical conductivity [EC], turbidity and Secchi depth [SD]), land use (built-up land, farmland, waters, forest, grassland, and unused land) in the watershed, and lake morphology (area and depth) on the composition and biomass of phytoplankton communities were assessed in 29 subtropical shallow lakes in Wuhan, China, during different seasons from December 2017 to November 2018. The results showed that phytoplankton in all 29 lakes was mainly composed of Cyanophyta, Chlorophyta, and Bacillariophyta. Phytoplankton abundance was highest in summer and lowest in winter. We analyzed the relative importance of the three groups of variables to the community composition of the phytoplankton by variance decomposition. The results showed that the three groups of environmental variables had the highest explanation rate (> 80%) for the composition of the phytoplankton community in summer and autumn, and the explanation rates in spring and winter were 42.1% and 39.8%, respectively. The water quality physicochemical parameters were the most important variables affecting the composition of phytoplankton communities, followed by land use in the watershed. Through generalized additive model and structural equation model analysis, we found that the land use and lake morphology had minimal direct impact on the Chl-a and cell density of phytoplankton, mainly by altering the TN, TP, turbidity, SD, DO, and EC, which indirectly affected phytoplankton. WT and nutrients were still the main predictors of phytoplankton abundance. Built-up land was the main source of nitrogen and phosphorus in lakes. Correlation analysis found that forest and grassland had positive impacts on reducing lake nitrogen and phosphorus contents. This showed that increasing grassland and forest in the watershed could reduce the pollutants entering the lake. Our findings will contribute to water quality management and pollution control for subtropical shallow lakes.

The response of phytoplankton communities to experimentally elevated temperatures in the presence and absence of Potamogeton crispus

Phytoplankton in shallow lakes will expectedly respond to climate warming and the response may differ between macrophyte and non-macrophyte lakes. To test the responses of phytoplankton to elevated temperature (+4.5 °C) at genus and functional group level, we conducted a 60-day experiment with and without the presence of Potamogeton crispus in the winter of Wuhan, China. When P. crispus was present, elevated temperature had little or no effect on the biomass and composition of the phytoplankton community. In the absence of P. crispus, however, elevated temperature significantly decreased the biomass and changed the composition of the phytoplankton. In contrast to many but not all published results mainly conducted during spring and summer, elevated temperature in winter reduced rather than increased the dominance of Cyanophyta. Instead, Chlorophyta became dominant. Redundancy analyses (RDA) showed that conductivity, light intensity and the ammonium/nitrate ratio (NH4:NO3) were the most important environmental variables, accounting for almost similar parts of the variations recorded in both genus and functional group composition. Among treatments, functional models did not show advantages over taxonomic models in depicting the response of the phytoplankton assemblage to the environmental conditions prevailing in our study conducted in winter, characterised by low phytoplankton abundance.

Effects of omnivorous tilapia on water turbidity and primary production dynamics in shallow lakes: implications for ecosystem management

The introduction of omnivorous tilapia into a variety of aquatic systems worldwide has led to a number of serious ecological problems. One of the main issues is an increase in water turbidity, which affects not only light penetration but also primary production and the distribution of phytoplankton and benthic algae in shallow lakes. These changes cause deterioration of water quality in these lakes. A 12-week mesocosm experiment was set up to test the hypotheses that omnivorous Nile tilapia (Oreochromis niloticus) introduced to a shallow water system will increase turbidity and nutrient levels in the water column and thereby boost growth of phytoplankton and depress benthic algae. Relative to the control treatments, the presence of tilapia led to higher concentrations of total nitrogen and total phosphorus in the water column, greater biomass of phytoplankton as measured by chlorophyll a (Chl a), greater concentrations of total suspended solids and inorganic suspended solids, lower light intensity, and lower biomass of benthic algae at the sediment surface. A tube-microcosm experiment using 32P radio tracer indicated that the presence of tilapia accelerated the release of sediment phosphorus (P) into the water column. We conclude that these invasive omnivorous fish not only stimulate growth of phytoplankton in shallow lakes by increasing nutrients in water column, but also depress benthic algal growth by promoting sediment resuspension, leading to increased turbidity of the water. Thus the removal of tilapia could be a useful practice for managers of shallow aquatic ecosystems, promoting benthic primary production and improving water clarity.

Interactions between bivalves and zooplankton: competition or intraguild predation? Implications for biomanipulation in subtropical shallow lakes

Zooplankton and bivalves are the main consumers of phytoplankton in shallow lakes; however, knowledge regarding trophic interactions between them is scarce. Competition for resources appears to be an obvious direct interaction, but the scheme may be more complex. Bivalves can consume all or only part of the zooplankton, constituting an intraguild predation module. In the present study we investigated the interaction between bivalves and zooplankton and its effects on phytoplankton grazing and community structure using an experimental approach in an aquarium. Two bivalve species were considered, namely Diplodon parallelopipedon (native) and Corbicula fluminea (non-native), in addition to a natural zooplankton community dominated by small-sized zooplankton. The findings indicate that phytoplankton consumption by the zooplankton is substantially less than that by the bivalves. Under the experimental conditions, the bivalves actively consumed small-sized zooplankton (rotifers and nauplii), but no consumption of medium-sized individuals (cladocerans and copepods) was observed. The differential consumption may have implications on the size and structure of zooplankton, favouring an average larger-sized community. Thus, phytoplankton may be negatively affected both directly by consumption and indirectly because of the resulting structure of the zooplankton community. The main results of the study are also discussed from the perspective of biomanipulation techniques in tropical and subtropical regions.

Forecasting Bifurcations from Large Perturbation Recoveries in Feedback Ecosystems.

Forecasting bifurcations such as critical transitions is an active research area of relevance to the management and preservation of ecological systems. In particular, anticipating the distance to critical transitions remains a challenge, together with predicting the state of the system after these transitions are breached. In this work, a new model-less method is presented that addresses both these issues based on monitoring recoveries from large perturbations. The approach uses data from recoveries of the system from at least two separate parameter values before the critical point, to predict both the bifurcation and the post-bifurcation dynamics. The proposed method is demonstrated, and its performance evaluated under different levels of measurement noise, with two ecological models that have been used extensively in previous studies of tipping points and alternative steady states. The first one considers the dynamics of vegetation under grazing; the second, those of macrophyte and phytoplankton in shallow lakes. Applications of the method to more complex situations are discussed together with the kinds of empirical data needed for its implementation.

Total phosphorus thresholds for regime shifts are nearly equal in subtropical and temperate shallow lakes with moderate depths and areas

Summary Published research suggests that the total phosphorus (TP) thresholds for the regime shifts between a clear‐water state dominated by submersed macrophytes and a turbid‐water state dominated by phytoplankton in shallow lakes vary with forms of lake basins and climates. However, such hypotheses remain untested by direct field evidence. We therefore tested the hypotheses with empirical data from subtropical lakes on the Yangtze floodplain and also from other lakes in temperate to tropical zones. TP thresholds were found to vary little at moderate depths, but to decrease notably when depth exceeds a level of probably 3–4 m, and increase sharply when depth is below a level of around 1–2 m. TP thresholds were found to be nearly equal in shallow lakes (1–2 m <mean depth <3–4 m; c. 0.1 km2 <area <at least 350 km2) from temperate to subtropical (probably to tropical) zones, being 80–120 mg m−3 for the forward shift from a clear‐water state to a turbid‐water state and 40–60 mg m−3 for the backward shift. The threshold of turbidity for the forward shift was found to be higher than that for the backward shift, amending the previous hypothesis of the equality of turbidity thresholds for both shifts. Our findings suggest that according to the subequality of TP thresholds, similar target concentrations for in‐lake TP can be set in most shallow lakes world‐wide to mitigate eutrophication.

Ambiguous climate impacts on competition between submerged macrophytes and phytoplankton in shallow lakes

1. Shallow lakes may switch from a state dominated by submerged macrophytes to a phytoplankton-dominated state when a critical nutrient concentration is exceeded. We explore how climate change may affect this critical nutrient concentration by linking a graphical model to data from 83 lakes along a large climate gradient in South America. 2. The data indicate that in warmer climates, submerged macrophytes may tolerate more underwater shade than in cooler lakes. By contrast, the relationship between phytoplankton biomass [approximated by chlorophyll-a (chl-a) or biovolume] and nutrient concentrations did not change consistently along the climate gradient. In warmer climates, the correlation between phytoplankton biomass and nutrient concentrations was overall weak, especially at low total phosphorus (TP) concentrations where the chl-a/?TP ratio could be either low or high. 3. Although the enhanced shade tolerance of submerged plants in warmer lakes might promote the stability of their dominance, the potentially high phytoplankton biomass at low nutrient concentrations suggests an overall low predictability of climate effects. 4. We found that near-bottom oxygen concentrations are lower in warm lakes than in cooler lakes, implying that anoxic P release from eutrophic sediment in warm lakes likely causes higher TP concentrations in the water column. Subsequently, this may lead to a higher phytoplankton biomass in warmer lakes than in cooler lakes with similar external nutrient loadings. 5. Our results indicate that climate effects on the competitive balance between submerged macrophytes and phytoplankton are not straightforward

Seasonal dynamics of macrophytes and phytoplankton in shallow lakes: a eutrophication‐driven pathway from plants to plankton?

Summary1. Seasonal relationships between macrophyte and phytoplankton populations may alter considerably as lakes undergo eutrophication. Understanding of these changes may be key to the interpretation of ecological processes operating over longer (decadal‐centennial) timescales.2. We explore the seasonal dynamics of macrophytes (measured twice in June and August) and phytoplankton (measured monthly May–September) populations in 39 shallow lakes (29 in the U.K. and 10 in Denmark) covering broad gradients for nutrients and plant abundance.3. Three site groups were identified based on macrophyte seasonality; 16 lakes where macrophyte abundance was perennially low and the water generally turbid (‘turbid lakes’); 7 where macrophyte abundance was high in June but low in August (‘crashing’ lakes); and 12 where macrophyte abundance was high in both June and August (‘stable’ lakes). The seasonal behaviour of the crashing and turbid lakes was extremely similar with a consistent increase in nutrient concentrations and chlorophyll‐a over May–September. By contrast in the stable lakes, seasonal changes were dampened with chlorophyll‐a consistently low (<10–15 μg L−1) over the entire summer. The crashing lakes were dominated by one or a combination of Potamogeton pusillus, Potamogeton pectinatus and Zannichellia palustris, whereas Ceratophyllum demersum and Chara spp. were more abundant in the stable lakes.4. A long‐term loss of macrophyte species diversity has occurred in many shallow lakes affected by eutrophication. One common pathway is from a species‐rich plant community with charophytes to a species‐poor community dominated by P. pusillus, P. pectinatus and Z. palustris. Such compositional changes may often be accompanied by a substantial reduction in the seasonal duration of plant dominance and a greater tendency for incursions by phytoplankton. We hypothesise a slow‐enacting (10–100 s years) feedback loop in nutrient‐enriched shallow lakes whereby increases in algal abundance are associated with losses of macrophyte species and hence different plant seasonal strategies. In turn such changes may favour increased phytoplankton production thus placing further pressure on remaining macrophytes. This study blurs the distinction between so‐called turbid phytoplankton‐dominated and clear plant‐dominated shallow lakes and suggests that plant loss from them may be a gradual process.

Limitations of using extracellular alkaline phosphatase activities as a general indicator for describing P deficiency of phytoplankton in Chinese shallow lakes

Extracellular phosphatase can be produced by phytoplankton to utilize organic phosphorus under phosphorus (P) deficiency. However, there is a controversy about its use as an indicator of P deficiency in natural phytoplankton community inferred by such an "induction-repression" mechanism. Size-fractionation of alkaline phosphatase activity (APA), soluble reactive phosphorus (SRP) concentration, algal density, and composition were determined in six Chinese shallow lakes ranking in gradient of trophic status, where a positive relationship between SRP concentration and algal density was observed. Enzyme-labeled fluorescence (ELF) method was used to localize phosphatase on cell membrane of algae. The so-called algal APA that associated with coarser particle (> 3.0 A mu m) accounted for the largest part of total APA. Within a lake with lower SRP concentration, the "induction-repression" mechanism held true. Contrastingly, both algal and total APA were positively related to SRP concentration based on the data across all study lakes with statistical significance, which may be explained firstly by algal composition. The lakes with higher SRP concentration were dominated by diatoms and green algae, while they easily produced extracellular phosphatases as evidenced by ELFA labeling. In parallel, the lakes with lower SRP concentration were dominated by cyanobacteria, while it was never ELFA-positive; secondly, ELFA-positive dots or structures suggested that, in lakes with higher trophic status, attached bacteria or heterotrophic microorganisms could substantially contribute to extracellular phosphatases for hydrolyzing organophosphoric compounds but probably utilizing the organic moiety as an organic carbon source. This process simultaneously produces inorganic P, leading to the co-occurrence of high phosphate concentration and APA. So, the contributor of APA are complex, which may produce extracellular phosphatase species-specific or not exclusively for P nutrient and consequently make it difficult to normalize APA with the exact biomass estimators. Therefore, it is not reasonable to use APA, normalized or not, as a general indicator for describing P deficiency of phytoplankton in shallow lakes especially eutrophic ones.

Can allelopathically active submerged macrophytes stabilise clear-water states in shallow lakes?

Inhibition of phytoplankton by allelochemicals released by submerged macrophytes is supposed to be one of the mechanisms that contribute to the stabilisation of clear-water states in shallow lakes. The relevance of this process at ecosystem level, however, is debated because in situ evidence is difficult to achieve. Our literature review indicates that allelopathically active species such as Myriophyllum, Ceratophyllum, Elodea and Najas or certain charophytes are among the most frequent submerged macrophytes in temperate shallow lakes. The most common experimental approach for allelopathic interference between macrophytes and phytoplankton has been the use of plant extracts or purified plant compounds. Final evidence, however, requires combination with more realistic in situ experiments. Such investigations have successfully been performed with selected species. In situ allelopathic activity is also influenced by the fact that phytoplankton species exhibit differential sensitivity against allelochemicals both between and within major taxonomic groups such as diatoms, cyanobacteria and chlorophytes. In general, epiphytic species apparently are less sensitive towards allelochemicals than phytoplankton despite living closely attached to the plants and being of key importance for macrophyte growth due to their shading. Light and nutrient availability potentially influence the sensitivity of target algae and cyanobacteria. Whether or not additional stressors such as nutrient limitation enhance or dampen allelopathic interactions still has to be clarified. We strongly propose allelopathy as an important mechanism in the interaction between submerged macrophytes and phytoplankton in shallow lakes based on the frequent occurrence of active species and the knowledge of potential target species. The role of allelopathy interfering with epiphyton development is less well understood. Including further levels of complexity, such as nutrient interference, grazing and climate, will extend this ecosystem-based view of in situ allelopathy.

The influence of different disturbance frequencies on the species richness, diversity and equitability of phytoplankton in shallow lakes

The influence of different disturbance frequencies on the species richness, diversity and equitability of phytoplankton in shallow lakes