Modeling cyanobacterial blooms in tropical reservoirs: The role of physicochemical variables and trophic interactions

Authors

Cihelio Alves Amorim, Ênio Wocyli Dantas, Ariadne do Nascimento Moura

Understanding the importance of environmental variables on the dominance of cyanobacteria is crucial for appropriately managing water resources. Although studies about temperate and subtropical regions show a high influence of nutrients and temperature on blooms, this relationship is still unclear for the tropics. Accordingly, we hypothesized that nutrients and temperature are the main factors driving cyanobacterial blooms in tropical reservoirs, and those relationships are intensified by the zooplankton. To test these hypotheses, we constructed a structural equation model based on the monitoring of ten reservoirs from Northeast Brazil. We analyzed the effects of physicochemical variables and zooplankton on cyanobacterial blooms and the biomass of four morphotypes. Cyanobacterial biomass varied within the reservoirs, with bloom records (0.2–268.4 mg L⁻¹) in all of them, primarily constituted by the colonial morphotype, followed by picocyanobacteria, heterocyted, and non-heterocyted filaments. The cyanobacterial community was driven mainly by chemical variables (55.14% of the variation), followed by physical (48.28%), and zooplankton (39.47%). Through the structural equation model, we demonstrated that total cyanobacterial biomass, as well as the morphotypes, were mainly influenced by omnivorous crustaceans and total dissolved phosphorus. Solar radiation, air temperature, mixing zone, and salinity were important to explain the biomass of the morphotypes. The model explained most of the variation in the picocyanobacterial blooms (79.8%), followed by total cyanobacteria (62.4%), heterocyted filaments (59.1%), non-heterocyted filaments (58.2%), and coccoids (55.1%). Zooplankton groups were also influenced by the physicochemical variables, which presented direct and indirect effects on cyanobacteria. Given the predictions of increased eutrophication, warming, and salinization, cyanobacterial blooms will become more intense in tropical reservoirs. Thus, restoring measures must be adopted to reduce bloom development, such as external phosphorus and salt loadings, and biomanipulation.

Trophic control changes with season and nutrient loading in lakes

Authors

Tanya L. Rogers Stephan B. Munch Simon D. Stewart Eric P. Palkovacs Alfredo Giron‐Nava Shin‐ichiro S. Matsuzaki Celia C. Symons

Experiments have revealed much about top‐down and bottom‐up control in ecosystems, but manipulative experiments are limited in spatial and temporal scale. To obtain a more nuanced understanding of trophic control over large scales, we explored long‐term time‐series data from 13 globally distributed lakes and used empirical dynamic modelling to quantify interaction strengths between zooplankton and phytoplankton over time within and across lakes. Across all lakes, top‐down effects were associated with nutrients, switching from negative in mesotrophic lakes to positive in oligotrophic lakes. This result suggests that zooplankton nutrient recycling exceeds grazing pressure in nutrient‐limited systems. Within individual lakes, results were consistent with a ‘seasonal reset’ hypothesis in which top‐down and bottom‐up interactions varied seasonally and were both strongest at the beginning of the growing season. Thus, trophic control is not static, but varies with abiotic conditions – dynamics that only become evident when observing changes over large spatial and temporal scales.

Effect of eutrophication and humification on nutrient cycles and transfer efficiency of matter in freshwater food webs

Authors

Maciej Karpowicz, Piotr Zieliński, Magdalena Grabowska, Jolanta Ejsmont-Karabin, Joanna Kozłowska & Irina Feniova

We evaluated how eutrophication and humification impacted nutrient cycles and the efficiency of carbon transfer in the planktonic food webs of 12 lakes in north-eastern Poland that differed in trophic state. Our results indicated that the effectiveness of carbon transfer between phytoplankton and zooplankton varied from 0.0005% to 0.14%, which is much lower than the theoretical 10%. The highest efficiency of carbon transfer occurred in the mesotrophic lakes due to the higher hypolimnetic zooplankton production, while the lowest efficiency was observed in the dystrophic lakes and in one eutrophic lake that was dominated by cyanobacteria. Inedible algae (e.g. Gonyostomum semen) and cyanobacteria appeared to be the main factors reducing the efficiency of the transfer of matter in pelagic food webs. The results of our study showed that plankton communities are a key component of the nutrient cycle in freshwater food webs. Phytoplankton were a very effective nitrogen sink, and in the mesotrophic lakes, up to 99% of the total nitrogen was sequestrated in phytoplankton. As a result, there was a depletion of inorganic nitrogen in the upper water layer. Furthermore, zooplankton were an important phosphorus sink, thus significantly influencing the nutrient cycles.

Effect of eutrophication and humification on nutrient cycles and transfer efficiency of matter in freshwater food webs

Authors

Maciej Karpowicz, Piotr Zieliński, Magdalena Grabowska, Jolanta Ejsmont-Karabin, Joanna Kozłowska & Irina Feniova

We evaluated how eutrophication and humification impacted nutrient cycles and the efficiency of carbon transfer in the planktonic food webs of 12 lakes in north-eastern Poland that differed in trophic state. Our results indicated that the effectiveness of carbon transfer between phytoplankton and zooplankton varied from 0.0005% to 0.14%, which is much lower than the theoretical 10%. The highest efficiency of carbon transfer occurred in the mesotrophic lakes due to the higher hypolimnetic zooplankton production, while the lowest efficiency was observed in the dystrophic lakes and in one eutrophic lake that was dominated by cyanobacteria. Inedible algae (e.g. Gonyostomum semen) and cyanobacteria appeared to be the main factors reducing the efficiency of the transfer of matter in pelagic food webs. The results of our study showed that plankton communities are a key component of the nutrient cycle in freshwater food webs. Phytoplankton were a very effective nitrogen sink, and in the mesotrophic lakes, up to 99% of the total nitrogen was sequestrated in phytoplankton. As a result, there was a depletion of inorganic nitrogen in the upper water layer. Furthermore, zooplankton were an important phosphorus sink, thus significantly influencing the nutrient cycles.

Grazing efficiency asymmetry drives zooplankton top-down control on phytoplankton in a subtropical lake dominated by non-toxic cyanobacteria

Authors

Lorena Pinheiro-Silva, Andros Tarouco Gianuca, Mônica Hessel Silveira & Mauricio Mello Petrucio

Body size is considered a powerful trait that impacts ecosystem processes and mediates species responses to environmental change. Diversity metrics informed by body size could thus predict ecosystem functions better than taxonomic diversity. However, despite its overwhelming power, the mechanisms by which size drives functions in freshwater ecosystems remain poorly known. Two mechanisms are often hypothesized to explain the relationship between diversity and ecosystem functioning: selection effects and complementarity. Here, we evaluate the relative importance of size-based and taxon-based approaches in explaining the strength of top-down control on phytoplankton and also aimed at disentangling the mechanism by which size operates. We found that size significantly explained the variation in top-down control, but size together with taxonomic diversity provided better predictions. The latter result differs somewhat from those reported in temperate regions and potentially reflects the limited size range that is typical of warmer waters. Our results also reveal the importance of selection effects relative to complementarity as a driver of top-down control. We highlight the importance of considering multiple aspects of biodiversity and recommend that metrics based on body size should complement, rather than replace, metrics of taxonomic diversity to predict ecosystem functions.

 

Temperature and the size of freshwater phytoplankton

Authors

Tamar Zohary, Giovanna Flaim & Ulrich Sommer

We review the literature on the relationship between water temperature and size of freshwater phytoplankton, to examine the hypothesis that freshwater phytoplankton, like marine phytoplankton and many other groups of organisms, conform to Bergmann’s Rule and become smaller with warming. We provide both experimental and field evidence in support of the above hypothesis, much of this evidence was hidden in studies focused on other issues, but presenting temperature and phytoplankton size data. Freshwater phytoplankton size shrinks with increasing temperature at both the species level (by cells or colonies becoming smaller) and at the community level (shift to smaller species). Exceptions to the Rule do occur but in most cases those exceptions can be explained by indirect effects of temperature on phytoplankton size, via processes such as grazing or nutrient availability. With global warming, freshwater phytoplankton are likely to be of smaller size. This article is dedicated to Colin S. Reynolds, who has had a leading role in our personal education and understanding of phytoplankton ecology.

 

Lack of dormancy to protect diversity: Decrease in diversity of active zooplankton community observed in lake with depauperate egg bank

Authors

Lauren N.Patterson, B. Dani Harris, Joseph A. Covi

The study of zooplankton communities in freshwater resources under anthropogenic pressures rarely includes the simultaneous assessment of dormant embryos in bottom sediments and active life-stages in the water column. A coastal lake with a history of coal-ash contamination and disruption by hurricanes provided an ideal opportunity to demonstrate the power of examining both dormant and active zooplankton. The primary objective of this study was to evaluate changes in structure of a multicellular zooplankton community that is under simultaneous pressure from anthropogenic pollution and hurricane-induced flooding. To evaluate change in community structure, the active zooplankton community in 2015 was compared to that observed in 1985. Shannon-Wiener and Simpson indices demonstrate that diversity of the active zooplankton community decreased during this 30-year span. In total, 31% of zooplankton species were lost, and new colonization accounts for 27% of species richness. Dominant species of all major taxonomic groupings changed. Because most zooplankton in freshwater lakes depend on dormant embryos to reestablish active populations after major disruptions, dormant embryos in the sediment “egg bank” were also quantified. Dormant cladoceran ephippia are present in bottom sediments, but dormant copepods and rotifers are missing. The existence of a dormant egg bank that is less diverse than the active community in a freshwater lake is unprecedented, and a depauperate “egg bank” would certainly impair community recovery after severe flooding from hurricanes. It is argued that a paradigm shift is needed in the ecological assessment of inland lakes in order to account for the critical role that dormant embryos (egg banks) play in freshwater zooplankton communities. Two challenges to achieving this are that 1. long-term monitoring is expensive and 2. data on dormant zooplankton are rarely available. This study provides an example of how to conduct such studies by leveraging historic data when long-term monitoring is not possible.

Sampling effects drive the species–area relationship in lake zooplankton

Authors

Leana D. Gooriah, Jonathan M. Chase

The island species–area relationship (ISAR) describes how the numbers of species increases with increasing size of an island (or island‐like habitat, such as lakes), and is one of the oldest laws in ecology. Despite its conceptual importance, there remains a great deal of ambiguity regarding the ISAR and its underlying processes. We compiled data from sampled zooplankton assemblages from several hundred lakes in North America and Europe to examine the influence of the three main hypothesized mechanisms leading to ISARs – passive sampling, disproportionate effects and habitat heterogeneity. We compiled data on lake zooplankton assemblages that reported sample‐level and lake level species richness estimates, as well as relative abundance data. In both North American and European lakes, we found a consistent and strong increase in total species richness with increasing lake area. However, when we compared the number of species standardized by number of individuals, there was no relationship between lake area and sample‐level species richness or an estimate of species relative abundances, calculated as the probability of interspecific encounter (PIE; a measure of evenness). This was true even when multiple samples were taken across lakes and combined, reducing the likelihood that habitat heterogeneity was driving the results. Overall, our results suggest that the ISAR of zooplankton in these lakes was most likely determined by sampling effects rather than disproportionate effects or habitat heterogeneity leading to more species in larger lakes. Understanding the mechanisms driving ISAR results such as ours can also help us develop predictions for biodiversity change when the area of these habitats changes.

Zooplankton invasions in the early 21st century: a global survey of recent studies and recommendations for future research

Authors

Eric Dexter, Stephen M. Bollens

We present a comprehensive survey of the scientific literature pertaining to non-indigenous and invasive zooplankton published across the first decades of the twenty-first century (i.e., 2000–2018). We provide a concise summary of the manner in which the scientific community has allocated its efforts to this issue in recent decades, and to illuminate trends that emerge from the literature. Our search yielded 620 publications encompassing 139 invasive zooplankton species, with invasive zooplankton reported from every region of the planet—including the Arctic and Antarctic. Most taxa were reported in a small number of publications, with the majority being mentioned in only a single paper. In contrast, approximately half of the surveyed publications concerned just four species: Bythotrephes longimanus, Mnemioposis leidyi, Cercopagis pengoi, and Daphnia lumholtzi. Our survey reveals strong geographic patterns among the literature, with most publications arising from economically developed western nations. We found that the majority of publications pertained to holoplanktonic organisms from freshwater habitats, especially from the North American Great Lakes. Based on these results, we present several recommendations for future research topics that may hold considerable opportunity for growth in our understanding of the invasion process.

Planktonic indicators of trophic states for a shallow lake (Baiyangdian Lake, China)

Authors

Caihong Tang, Yujun Yi, Zhifeng Yang, Yang Zhou, Teklit Zerizghi, Xuan Wang, Xiuli Cui, Pengyu Duan

Eutrophication in lake ecosystems poses a serious threat to the water quality and function of aquatic ecosystems. The species composition and community structure of plankton change directly with variations in lake trophic states. Seasonal sample collections of phytoplankton, zooplankton and environmental variables were conducted in spring, summer, and fall from a large shallow lake, Baiyangdian Lake, China. The species richness, community composition and temporospatial variations of phytoplankton and zooplankton were analyzed. The lake trophic states were assessed using the comprehensive trophic state index (CTSI) and rotifer trophic state index (TSI_ROT). The results indicated that 69.1% of the lake area showed slight eutrophication, 29.3% showed mesotrophication, and 1.6% showed moderate eutrophication. The Shannon-Wiener diversity index and Pielou evenness index were employed to assess the community diversity of phytoplankton and zooplankton. The dominant taxa and most dominant species of phytoplankton and zooplankton were determined; Cyanophyta and Rotifera were the dominant phytoplankton and zooplankton taxa, respectively. Microcystis sp. and Polyarthra vulgaris were the most dominant species of phytoplankton and zooplankton, respectively. Redundancy analysis (RDA) was applied to identify the key environmental variables that influenced the species, diversity indices and species abundance of phytoplankton and zooplankton. The results showed that ammonia nitrogen (NH₄⁺), total nitrogen (TN), total phosphorus (TP), and dissolved oxygen (DO) were the main environmental factors influencing the species abundance and diversity. The current lake plankton species number and diversity index were lower than those in past decades, as determined by comparing the community characteristics of phytoplankton and zooplankton with historical records. Comparisons of TN:TP ratios with those of other lakes suggested that nitrogen was the limiting nutrient for lake eutrophication. Baiyangdian Lake could have a high potential for eutrophication based on the environmental and ecological characteristics of the lake.