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Despite the lower fish biomass in tropical lakes, no significant difference was found in either zooplankton or phytoplankton communities or in the zooplankton to phytoplankton biomass ratio between the two sets of lakes. In agreement with their hypothesis, the between-lake and diversities from phytoplankton and zooplankton were generally negatively related to EH, whereas the plankton diversity showed an opposite pattern at both within-lake and between-lake scales. Introduction The concept of ecosystems is one of the most important within ecology, and ecosystem ecology has become a significant paradigm (sensu Kuhn 1962) for studying ecological systems. We hope that this special issue will improve the understanding of these important ecosystems and also challenge shallow lakes ecologists to consider in their studies the relationships between spatial patterns and ecological processes at spatial scales that extend well beyond the individual lake. [Extracted from the article]

Although considered a key functional trait, little is known about how zooplankton feeding mode affects top‐down regulation of phytoplankton communities. Indeed, copepods are expected to promote the dominance of toxic phytoplankton by selective removal of their edible competitors; however, empirical evidence comparing the effect among calanoid and cyclopoid copepods is lacking.We compared the top‐down effects of two copepods with contrasting feeding modes—the calanoid Notodiaptomus iheringi (current feeder) and the cyclopoid Thermocyclops decipiens (ambush feeder) — on the relative and absolute biomass of the filamentous cyanobacterium Raphidiopsis raciborskii co‐cultured with the nutritious eukaryotic phytoplankton Cryptomonas obovata in a week‐long laboratory assay.The current feeder had a stronger top‐down effect on the biomass of both prey throughout the experiment, with mass‐specific clearance rates 3–5× higher than ambush feeder. By the end of the experiment, the current feeder significantly reduced cyanobacteria biomass compared to controls while the ambush feeder did not. During the week‐long experiment, the current feeder switched from grazing on edible prey to cyanobacteria as the former became less abundant.Contrary to expectation, neither of the copepod species promoted cyanobacterial dominance by the end of the experiment. This is because both grazers, but especially the current feeder, initially increased but subsequently decreased the relative contribution of cyanobacteria to total phytoplankton biomass. Moreover, both copepods decreased the length of cyanobacteria filaments by c. 70%Current feeders can switch from edible prey to cyanobacteria when the abundance of shortened filaments surpasses the abundance of edible prey. While top‐down regulation of phytoplankton can be stronger for current feeding copepods, ambush feeding copepods can have a significant role during blooms by shortening cyanobacterial filaments. Hence, the broader role of contrasting copepod feeding traits on phytoplankton communities merits further study. [ABSTRACT FROM AUTHOR]

The tropical copepod Notodiaptomus iheringi ( N. iheringi) is an ideal subject for studying zooplankton responses to cyanobacteria because it co-exists with permanent blooms across widespread regions in South America in high abundance. Single and mixed diets containing Cryptomonas and either a microcystin-producing (MC+) or microcystin-lacking (MC−) Microcystis were offered to N. iheringi at different proportions in a 10-day laboratory survival test to distinguish between the effects of toxicity versus nutrition. As expected, the pure MC+ Microcystis diet caused acute toxicity, indicated by high mortality compared to starved copepods. Both Microcystis strains were ingested in a 3-h short-term grazing experiment with pure diets. Despite its toxicity as the sole food source, survival was unaffected by MC+ Microcystis in mixed food diets. Even when MC+ Microcystis was 90% of the total food, survival was similar to the control with 10% Cryptomonas only. Hence, the survival in mixed food diets was controlled by the amount of Cryptomonas, not Microcystis. Previous reports show strong negative effects of Microcystis on copepod survival despite abundant high-quality food. Although this is the first example of copepods avoiding acute Microcystis toxicity in mixed diets, it could be a common trait where permanent blooms dominate the ecosystem. [ABSTRACT FROM AUTHOR]

We investigated the role of filament length of the toxic cyanobacterium Cylindrospermopsis raciborskii on the grazing of Daphnia magna by providing different food mixtures. It was hypothesized that longer C. raciborskii filaments would reduce the clearance rates of D. magna more than shorter ones. Also, C. raciborskii was expected to have a stronger negative impact on the grazing of larger animals. The clearance rates of two D. magna size classes (smaller, 2.0±0.1 mm and larger, 3.1±0.06 mm) were measured in short-term laboratory grazing experiments using individuals fed with two different monocultures of a saxitoxin producer C. raciborskii strain distinguished by filament lengths (average 137 and 61 µm). The cyanobacteria were offered as the sole food source and as a mixture (1:1) with Scenedesmus obliquus. A treatment with only S. obliquus was also applied. The length of C. raciborskii filaments did not have a clear influence on the clearance rates of D. magna within the range we tested. Thus, longer C. raciborskii filaments may not necessarily cause stronger feeding inhibition than shorter ones. Most of the grazing reduction occurred when C. raciborskii was the sole food for either size of D. magna. Larger animals experienced increased feeding inhibition by the addition of C. raciborskii to the diet. The proportion of C. raciborskii relative to suitable food may be a key factor for the outcome of feeding performance of D. magna. We encourage other authors to test the validity of these conclusions over a larger range of filament lengths and on strains with different biochemical makeup. [ABSTRACT FROM AUTHOR]

We tested the hypothesis that calanoid copepods would adapt to extended periods of Microcystis exposure by increasing selective feeding on alternative food. Copepod (Eudiaptomus gracilis) clearance rates were compared before and after a 5-d acclimation to Microcystis aeruginosa using paired food mixtures containing a microcystin-producing (MC+) or -lacking (MC-) strain and the green alga Chlamydomonas reinhardtii. Acclimation reduced the ingestion of Microcystis, increased ingestion of Chlamydomonas, and subsequently increased feeding selectivity. The effect of acclimation was more pronounced for food mixtures with MC+ Microcystis, suggesting that E. gracilis uses a strain-specific cue related to microcystin or microcystin itself to avoid harmful food. The results indicate that exposure to sublethal abundances of Microcystis may increase copepod tolerance to blooms, given sufficient alternative food. Zooplankton grazing behavior should, therefore, be viewed as flexible and adaptive to extended periods of cyanobacteria blooms. [ABSTRACT FROM AUTHOR]

The tolerance traits of zooplankton co-occurring with toxic cyanobacteria are poorly understood. To evaluate the tolerance mechanisms of Notodiaptomus iheringi, an abundant tropical copepod coexisting with toxic cyanobacteria blooms across South America, grazing and egg production rates were measured in the laboratory using single- and mixed-food diets containing either a microcystin producing (MC+) or lacking (MC-) strain of Microcystis with Cryptomonas as control food. Pure Microcystis diets resulted in no egg production. In mixed-food diets, the MC+ strain had no effect, while the MC-strain increased egg production relative to the controls. Clearance rates were highest on Cryptomonas, intermediate on the MC-strain and lowest on the MC+ Microcystis. Notodiaptomus iheringi selectively avoided Microcystis in mixed diets. Total grazing was not inhibited by Microcystis abundance as long as Cryptomonas was available. Egg production was unaffected by the ingestion of Microcystis but depended on the abundance and ingestion of Cryptomonas. Consequently, both selective grazing on nutritious food and the detoxification of ingested cyanobacteria may be critical traits for N. iheringi to tolerate blooms of toxic cyanobacteria. The results contrast with reports showing strong negative effects of Microcystis on zooplankton fitness and highlight the diversity in zooplankton tolerance traits. [ABSTRACT FROM AUTHOR]

While warming and eutrophication have increased the frequency and magnitude of harmful cyanobacterial blooms globally, the scenario for many eutrophic tropical freshwaters is a perennial year-round bloom. Yet, the drivers of persistent blooms are less understood when conditions such as light, temperature, and nutrients favor cyanobacteria growth year-round, and especially in regions facing recurrent periods of drought. In order to understand the drivers of cyanobacteria dominance, we assessed the abiotic conditions related to the abundance and dominance of the two dominant bloom-forming genera Raphidiopsis and Microcystis, in six shallow, man-made lakes located in the semiarid Northeastern region of Brazil during a prolonged regional drought. Lower water level corresponded to increased phosphorous and nitrogen concentration and, consequently, phytoplankton biomass. Cyanobacterial biomass was also proportional to phosphorus concentrations during year-round blooms. Yet, the two dominant cyanobacterial genera, Raphidiopsis and Microcystis, seldom co-occurred temporally and the switch between them was driven by water transparency. Our results illustrate the effects of drought induced water level reductions on the biomass and composition of cyanobacterial blooms in tropical shallow man-made lakes. Given the ideal year-round conditions (i.e., high light and temperature), droughts may be expected to intensify the risk and multitude of problems associated with eutrophication. [ABSTRACT FROM AUTHOR]

Lucrécia city, known to harbor a high cancer rate, is located in a semiarid region characterized by the presence of mineral reservoirs, facing a high exposure to metal and natural radioactivity. The present study aimed to assess the environmental scenario at a semiarid region located in Northeastern Brazil. Metal concentration, alpha and beta radiation, and cyanobacteria content in tap water along with indoor radon and gamma emitters (U, K and Th) concentrations were measured. In addition, mutagenic and nuclear instability effects were assessed using buccal micronucleus cytome assay. The study included five samplings corresponding to a period between 2007 and 2009. Drinking water from Lucrécia city presented levels of Mn, Ni and Cr along with cyanobacteria in concentrations one to four times higher than regulatory guidelines considered. Furthermore, high levels of all the tested radionuclides were found. A high percentage of the houses included in this study presented indoor radon concentrations over 100 Bq m −3 . The mean annual effective dose from Lucrécia houses was six times higher than observed in a control region. The levels of exposure in most of the Lucrécia houses were classified as middle to high. A significant mutagenic effect, represented as an increase of micronuclei (MN) frequency and nuclear abnormalities as nuclear buds (NB), binucleated cells (BN), and pyknotic cells (PYC) were found. The results obtained highlight the role of high background radioactivity on the observed mutagenic effect and could help to explain the exacerbated cancer rate reported in this locality. [ABSTRACT FROM AUTHOR]