A complete analysis of the macro‐ and microzooplankton of a monomictic lake indicates that Protozoa dominate the community numerically. During winter mixis, ciliates are found at densities of 1–8 × 10 3 ·liter −1 and constitute up to 32% of the zooplankton community biomass. With summer stratification crustaceans decline, while both the relative and absolute abundance of protozoans and rotifers increase. Densities of protozoans are highest (1–2 × 10 5 ·liter −1 ) in the metalimnion where scuticociliates bloom in zones of intense bacterial activity. During the period of these blooms (July–October), Protozoa account for 15–62% of the zooplankton biomass. This suggests that Protozoa make a significant contribution to rates of grazing, nutrient regeneration, and secondary productivity and should not be overlooked in zooplankton community studies.
Abstract An important tenet of science is establishing the reproducibility of findings. While long‐term studies may seem ill‐suited to this goal, here we provide an example of reproducible results from repeated nutrient additions to a lake. We added nitrogen and phosphorus to Peter Lake in 9 yr of a 33‐yr study. For seven of these nine additions, phytoplankton biomass, as measured by seasonal mean chlorophyll a , increased in proportion to the rate of nutrient loading. Additionally, for these seven additions, similar nutrient loading rates resulted in mean Chl a concentrations within a roughly twofold range—an outcome within expectation given uncontrolled sources of variation in a whole‐lake manipulation. However, for two of the nine nutrient additions, Chl a concentrations were well below expected concentrations. The low chlorophyll responses co‐occurred with years having the highest water color (absorbance of light at 440 nm). The number of years of nutrient additions was too limited to strongly test the influence of color at the scale of seasonal mean values. We, therefore, tested for the effect of phosphorus load and color, on Chl a using time series models of weekly data. At the weekly scale, there was a strong negative effect of color on chlorophyll concentration. Overall, the repeated nutrient additions provided a confirmation of existing models at the whole‐lake scale and demonstrated an interesting exception to these models. Including repeated manipulations as part of long‐term studies is an important way to test generalizations and to identify unexpected outcomes that raise new questions.
When it comes to evaluating lakes at regional and global scales, a key need is accurate estimates of the abundance and size‐distribution of lakes, which are usually described with the Pareto distribution. We demonstrate the considerable uncertainty that truncation in the lower tail of the Pareto distribution introduces into lake abundance estimates and the selection of the lake size‐distribution. Truncation in the lower tail eliminates lakes below a certain size and is generally performed because small lakes are not accurately represented on maps. When simulated data are truncated to mimic available lake size data, non‐Pareto distributions are visually and statistically indistinguishable from the Pareto distribution. The Pareto distribution may be one of many possible forms that mimic the global lake size‐distribution in the upper tail, but the fit of the Pareto to the lower tail is uncertain, largely because the abundance of small lakes is uncertain. Some other potential size‐distributions, such as the lognormal distribution, predict abundances of small lakes to be orders of magnitude lower than do the Pareto distribution predictions. Highly resolved regional lake size data for the Adirondack Mountains of New York and the Northern Highland Lake District of Wisconsin do not conform to the Pareto distribution. Lake sizes on Mars also do not conform to the Pareto. Uncertainty in the lake size‐distribution seriously limits understanding of the significance of lakes as repositories of organic carbon as well as the calculation of global greenhouse gas emissions from these systems.
Abstract Heatwaves are increasing in frequency, duration, and intensity in ocean, coastal, and lake ecosystems. While positive water temperature trends have been documented in many rivers, heatwaves have not been analyzed. This study examined heatwaves in rivers throughout the United States between 1996 and 2021. Riverine heatwaves increased in frequency over the study period, with the most robust increases occurring in summer and fall, in mid‐ to high‐order streams, and at free‐flowing sites and sites above a reservoir. The increase in heatwave frequency was accompanied by an increase in moderate strength heatwaves as well as a doubling of the annual mean total number of heatwave days at a site. Riverine heatwaves were often associated with normal or below‐normal discharge conditions and at sites with a mean annual discharge ≤ 250 m 3 s −1 . These results provide the first assessment of heatwaves in rivers for a large geographic area in the United States.
Aquatic foods are highly traded, with nearly 60 million tonnes exported in 2020, representing 11% of global agriculture trade by value. Despite the vast scale, basic characteristics of aquatic food trade, including species, origin, and farmed vs wild sourcing, are largely unknown due to the reporting of trade data. Consequently, we have a coarse picture of aquatic food trade and consumption patterns. Here, we present results from a database on species trade that aligns production, conversion factors, and trade to compute apparent consumption for all farmed and wild aquatic foods from 1996 to 2020. Over this period, aquatic foods became increasingly globalized, with the share of production exported increasing by 40%. Importantly, trends differ across aquatic food sectors. Global consumption also increased by 19.4% despite declining marine capture consumption, and some regions became increasingly reliant on foreign-sourced aquatic foods. To identify sustainable diet opportunities among aquatic foods, our findings, and underlying database enable a greater understanding of the role of trade in rapidly evolving aquatic food systems.
A Bayesian mixing model and stable isotopes of carbon, nitrogen, and hydrogen were used to evaluate the extent to which six consumers (three fishes, two zooplankton, and a snail) in a naturally productive lake used terrestrial resources, epilimnetic and metalimnetic phytoplankton, benthic algae, and macrophytes. Resource use varied with consumer habitat use and feeding ability, but allochthony was consistently low (averaging 15% among consumers). The pelagic invertebrates Skistodiaptomus oregonensis and Chaoborus spp. relied on phytoplankton from the epilimnion (59% and 49%, respectively) and to a lesser extent from the metalimnion (28% and 26%, respectively); terrestrial resources comprised 9% and 18% of the diet of these consumers, respectively. The snail Helisoma trivolvis relied mainly on littoral resources (floating‐leafed macrophytes; 68% of diet), but terrestrial resources also constituted a substantial portion of its diet (21%). The fishes integrated among habitats more evenly than the other consumers, but pelagic resources formed the largest portion of their diets ( Pimephales promelas = 64%, Lepomis gibbosus = 47%, and Perca flavescens = 47%). L. gibbosus was the fish with the most allochthonous diet (23%). The consumers of this productive lake were not highly dependent on allochthonous materials and tended to rely most heavily on local resources, including macrophytes.
Bacterial abundances, biomass, and production were measured over a 3‐yr period at stations along a 158‐km reach of the tidal, freshwater Hudson River. Bacterial abundances ranged from 1 to 10 × 10 6 cells ml −1 with maximal values in summer. Abundance and production averaged over all stations for the ice‐free season (April through December) were 4.9 and 9.1 × 10 9 cells liter −1 d −1 , respectively, and both were significantly correlated with temperature. Neither bacterial abundance nor production showed significant spatial variability over the study reach. In contrast to the results from many autotrophic ecosystems, annual average bacterial abundances from different stations were not significantly correlated with algal standing stocks, and bacterial production was only weakly related to rates of primary production. Absolute rates of bacterial C production were greater than phytoplankton primary production, indicating that much of the bacterial secondary production in this portion of the river must be supported by nonphytoplanktonic organic C.
Although both nutrient inputs and zooplankton grazing are important to phytoplankton and bacteria in lakes, controversy surrounds the relative importance of grazing pressure for these two groups of organisms. For phytoplankton, the controversy revolves around whether zooplankton grazers, especially large cladocerans like Daphnia, can effectively reduce phytoplankton populations regardless of nutrient conditions. For bacteria, little is known about the balance between possible direct and indirect effects of both nutrients and zooplankton grazing. However, there is evidence that bacteria may affect phytoplankton responses to nutrients or zooplankton grazing through direct or apparent competition. We performed a mesocosm experiment to evaluate the relative importance of the effects of nutrients and zooplankton grazing for phytoplankton and bacteria, and to determine whether bacteria mediate phytoplankton responses to these factors. The factorial design crossed two zooplankton treatments (unsieved and sieved) with four nutrient treatments (0, 0.5, 1.0 and 2.0 μg phosphorus (P) l−1 day−1 together with nitrogen (N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300 μm mesh reduced the average size of crustacean zooplankton in the mesocosms, decreased the numbers and biomass of Daphnia, and increased the biomass of adult copepods. Nutrient enrichment caused significant increases in phytoplankton chlorophyll a (4–5×), bacterial abundance and production (1.3× and 1.6×, respectively), Daphnia (3×) and total zooplankton biomass (2×). Although both total phytoplankton chlorophyll a and chlorophyll a in the <35 μm size fraction were significantly lower in unsieved mesocosms than in sieved mesocosms, sieving had no significant effect on bacterial abundance or production. There was no statistical interaction between nutrient and zooplankton treatments for total phytoplankton biomass or bacterial abundance, although there were marginally significant interactions for phytoplankton biomass <35 μm and bacterial production. Our results do not support the hypothesis that large cladocerans become less effective grazers with enrichment; rather, the difference between phytoplankton biomass in sieved versus unsieved zooplankton treatments increased across the gradient of nutrient additions. Furthermore, there was no evidence that bacteria buffered phytoplankton responses to enrichment by either sequestering P or affecting the growth of zooplankton.
We measured mortality of protozoans and rotifers in three lakes of contrasting zooplankton communities. We also compared protozoan growth in an experiment which controlled Daphnia biomass but varied body size. Mortality was determined as the difference between growth rates over 24 h in containers with and without zooplankton. Growth rates of heterotrophic flagellates and ciliates were high in the presence of a small assemblage of zooplankton and near zero or negative when either Daphnia pulex or Daphnia galeata was the dominant zooplankton species. Growth rates of rotifers were also usually lower in the presence of Daphnia. Mortality rates of heterotrophic flagellates, ciliates, and rotifers were positively related to the mean body size of Daphnia in comparisons among experiments. In an experiment with equal biomasses but different sizes of D. pulex, flagellate growth rates were lower in treatments with large Daphnia. High mortality in zooplankton communities dominated by larger species of Daphnia appears to be important in determining differences in the abundances of protozoans and rotifers among lakes.
