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Two invasive mussels (zebra mussel, Dreissena polymorpha and quagga mussel D. rostriformis bugensis) have restructured the benthic habitat of many water bodies in both Europe and North America. Quagga mussels dominate in most lakes where they co-occur even though zebra mussels typically invade lakes first. A reversal to zebra mussel over time has rarely been observed. Laboratory experiments have shown that quagga mussels grow faster than zebra mussels when predator kairomones are present and this faster growth is associated with lower investment in anti-predator response in quagga mussels than zebra mussels. This led to the hypothesis that the dominance of quagga mussels is due to faster growth that is not offset by higher vulnerability to predators when predation rates are low, as may be expected in newly colonized lakes. It follows that in lakes with high predation pressure, the anti-predatory investments of zebra mussels should be more advantageous and zebra mussels should be the more abundant of the two species. In Onondaga Lake, NY, a meso-eutrophic lake with annual mussel surveys from 2005 to 2018, quagga mussels increased from less than 6% of the combined mussel biomass in 2007 to 82% in 2009 (from 3 to 69% by number), rates typical of this displacement process elsewhere, but then declined again to 11-20% of the mussel biomass in 2016-2018. Average total mussel biomass also declined from 344-524 g shell-on dry weight (SODW)/m.sup.2 in 2009-2011 to 34-73 g SODW/m.sup.2 in 2016-2018, mainly due to fewer quagga mussels. This decline in total mussel biomass and a return to zebra mussel as the most abundant species occurred as the round goby (Neogobius melanostomus) increased in abundance. Both the increase to dominance of quagga mussels and the subsequent decline following the increase in this molluscivorous fish are consistent with the differences in the trade-off between investment in growth and investment in defenses of the two species. We predict that similar changes in dreissenid mussel populations will occur in other lakes following round goby invasions, at least on the habitats colonized by both species.

Since being introduced into the Laurentian Great Lakes in the 1990s, round goby ( Neogobius melanostomus ) has spread rapidly, reaching the Hudson River Estuary, NY in 2021. To address the expansion potential into saline environments from this North American coastal invasion front, we experimentally assessed the salinity tolerance of adult round gobies. Water temperatures vary widely in temperate aquatic ecosystems, and our study is the first to investigate the effect of temperature on the salinity tolerance of round goby, conducting tolerance trials at three temperatures: a preferred temperature of 20°C, 26°C reflective of summer conditions, and 5°C reflective of winter conditions. Adult round gobies were subjected to weekly salinity increases of 3 parts per thousand (ppt), concluding at 33ppt. Study specimens were monitored for stress cues (behavior changes and color changes), and mortality. We found significant salinity tolerance differences dependent on water temperature, with the highest tolerance at 5°C and the lowest tolerance at 26°C. By 30ppt, survival was 87% at 5°C and only 7% at 26°C. Based on mortality results, round goby expansion may occur year-round into brackish portions (<21ppt) of the Hudson River Estuary as far south as the Harlem River. This would open access to portions of Long Island Sound, potentially rendering other coastal watersheds to be at risk of invasion. However, temperature-dependent salinity tolerance findings suggest round goby expansion potential into high salinity habitats may be seasonally dependent, with expansion opportunities occurring in colder months and expansion barriers occurring in warmer months. To assess longer term survival and body condition, another experiment maintained round gobies at four sustained salinities (≤ 21ppt) for ten weeks at 20°C. Growth and hepatosomatic index at 21ppt were significantly lower (p<0.001) than at 1, 9, and 15ppt, indicating sustained exposure to higher salinities may affect energy stores, potentially limiting establishment potential.

Deep chlorophyll maxima (DCM) are common in stratified lakes and oceans, and phytoplankton growth within DCM often contributes significantly to total system production. Theory suggests that properties of DCM should be predictable by trophic state, with DCM becoming deeper, broader, and less productive with greater oligotrophy. However, rigorous tests of these expectations are lacking in freshwater systems. We use data generated by the U.S. EPA from 1996 to 2017, including in situ profile data for temperature, photosynthetically active radiation (PAR), chlorophyll, beam attenuation (c p), and dissolved oxygen (DO), to investigate patterns in DCM across lakes and over time. We consider trophic state, 1% PAR depth (z 1%), thermal structure, and degree of photoacclimation as potential drivers of DCM characteristics. DCM depth and thickness generally increased while DCM chlorophyll concentration decreased with decreasing trophic state index (greater oligotrophy). The z 1% was a stronger predictor of DCM depth than thermal structure. DCM in meso-oligotrophic waters were closely aligned with maxima in c p and DO saturation, suggesting they are autotrophically productive. However, the depths of these maxima diverged in ultra-oligotrophic waters, with DCM occurring deepest. This is likely a consequence of photoacclimation in high-transparency waters, where c p can be a better proxy for phytoplankton biomass than chlorophyll. Our results are generally consistent with expectations from DCM theory, but they also identify specific gaps in our understanding of DCM in lakes, including the causes of multiple DCM, the importance of nutriclines, and the processes forming DCM at higher light levels than expected.

Quantifying the relationship between phytoplankton and zooplankton may offer insight into zooplankton sensitivity to shifting phytoplankton assemblages and the potential impacts of producer-consumer decoupling on the rest of the food web. We analyzed 18 years (2001–2018) of paired phytoplankton and zooplankton samples collected as part of the United States Environmental Protection Agency (U.S. EPA) Great Lakes Biology Monitoring Program to examine both the long-term and seasonal relationships between zooplankton and phytoplankton across all five Laurentian Great Lakes. We also analyzed effects of phytoplankton diversity on zooplankton biomass, diversity, and predator-prey (zooplanktivore/grazer) ratios. Across the Great Lakes, there was a weak positive correlation between total algal biovolume and zooplankton biomass in both spring and summer. The relationship was weaker and not consistently positive within individual lakes. These trends were consistent over time, providing no evidence of increasing decoupling over the study period. Zooplankton biomass was weakly negatively correlated with algal diversity across lakes, whereas zooplankton diversity was unaffected. These relationships did not change when we considered only the edible phytoplankton fraction, possibly due to the high correlation between total and edible phytoplankton biovolume in most of these lakes. Lack of strong coupling between these producer and consumer assemblages may be related to lagging responses by the consumers, top-down effects from higher-level consumers, or other confounding factors. These results underscore the difficulty in predicting higher trophic level responses, including zooplankton, from changes in phytoplankton assemblages.

Gut content analyses have found that round gobies ( Neogobius melanostomus ) are highly dependent on dreissenid mussels but stable isotope analysis has often suggested that the dreissenid contribution is lower. However, estimation of dietary contributions with stable isotopes relies on accurate discrimination factors (fractionation factors). To test if discrimination values commonly used in aquatic food web studies are suitable for round gobies, we collected round gobies from Oneida Lake, raised them for 63 days under four different diets ( Chironomus plumosus , Mytilus chilensis , Dreissenia spp., Euphausia superba ) and measured the change in white muscle δ 13 C and δ 15 N. Gobies were also collected throughout Oneida Lake for gut content and stable isotope analysis. Diets changed as round gobies grew, with small round gobies (17-42mm) feeding mostly on cladocera and chironomids, intermediate sized gobies (43-94mm) transitioning from chironomid to dreissenid consumption, and larger gobies (95-120mm) predominantly consuming dreissenids, similar to findings in other studies. Discrimination factors were obtained by fitting a commonly used asymptotic regression equation describing changes in fish δ 13 C and δ 15 N as a function of time and diet stable isotope ratios. The discrimination factor determined for δ 13 C (-0.4‰ ± 0.32, SE) was lower than the “standard” value of 0.4‰, while that of δ 15 N (4.0‰ ± 0.32, SE) was higher than the standard value of 3.4‰. Turnover rates for both δ 13 C and δ 15 N were estimated as 0.02 ‰*day -1 . The use of experimentally determined discrimination factors rather than “standard” values resulted in model estimates that agree more closely with the observed increasing importance of dreissenids in gut content of larger gobies. Our results suggest that the importance of dreissenid mussels inferred from stable isotope studies may be underestimated when using standard isotopic discrimination values.

Freshwater lakes are severely threatened, due largely to excess inputs of nutrients and other contaminants. Phosphorus (P) is receiving renewed attention due to recent increases in toxic cyanobacteria blooms in lakes worldwide. We investigated groundwater seepage for its role in P loading dynamics at Oneida Lake, New York, USA—one of the most well-studied lakes globally. P loading was measured at representative sites along the 88 km shoreline over three summers by directly measuring groundwater flow using seepage meters and porewater samplers. Groundwater seepage was a continuous and significant source of dissolved P over the summer months, comparable to tributary sources to the lake during that time. This constant input has enriched the concentrations of P in the nearshore surface waters, significantly above levels in the pelagic zone. Pore Total Phosphorus (TP) concentrations and loads reached extremely high values (up to 100 mg/L), with inorganic P representing only ~ 10% of TP per site. Groundwater seepage flows and P loadings were highly variable across space and time, partially explained by adjacent land uses and precipitation. Our research concludes that groundwater seepage is a significant, but overlooked, source of dissolved P and a crucial factor driving summer primary production at Oneida Lake, and likely other temperate lakes.

Zebra mussels (Dreissena polymorpha) are known to increase water clarity and affect ecosystem processes in invaded lakes. During the last decade, the conspecific quagga mussels (D. rostriformis bugensis) have displaced zebra mussels in many ecosystems including shallow lakes such as Oneida Lake, New York. In this study, an eight-week mesocosm experiment was conducted to test the hypothesis that the displacement of zebra mussels by quagga mussels leads to further decreases in phytoplankton and increases in water clarity resulting in increases in benthic algae. We found that the presence of zebra mussels alone (ZM), quagga mussels alone (QM), or an equal number of both species (ZQ) reduced total phosphorus (TP) and phytoplankton Chl a. Total suspended solids (TSS) was reduced in ZM and ZQ treatments. Light intensity at the sediment surface was higher in all three mussel treatments than in the no-mussel controls but there was no difference among the mussel treatments. There was no increase in benthic algae biomass in the mussel treatments compared with the no-mussel controls. Importantly, there was no significant difference in nutrient (TP, soluble reactive phosphorus and NO3-) levels, TSS, phytoplankton Chl a, benthic algal Chl a, or light intensity on the sediment surface between ZM, QM and ZQ treatments. These results confirm the strong effect of both mussel species on water clarity and indicate that the displacement of zebra mussel by an equivalent biomass of quagga mussel is not likely to lead to further increases in water clarity, at least for the limnological conditions, including summer temperature, tested in this experiment.

Invasion of non‐native species might alter food web structure and the strength of top‐down control within lake ecosystems. As top‐down control exerted by fish populations is often dominated by young of the year fish, the impact of new fish species might depend on the feeding rates of the juvenile fish. Here we provide comparative analyses of feeding rates of juvenile whitefish (Coregonus wartmanni) – a native and specialised planktivore and an invasive generalist (sticklebacks, Gasterosteus aculeatus). We studied feedings rates of whitefish and sticklebacks in aquaria experiments using 2 cm to 8 cm fish feeding on seven zooplankton species common to Lake Constance. As whitefish hatch several months earlier than sticklebacks, 0+ whitefish are larger than 0+ sticklebacks throughout the year and hence are predicted to have higher feeding rates on especially large zooplankton species. We show that sticklebacks as small as 2 cm were able to feed on the largest zooplankton species of Lake Constance. Further, stickleback feeding rates were similar to both the same size 0+ whitefish and the larger 0+ whitefish co‐occurring with smaller 0+ sticklebacks. Hence, 0+ sticklebacks will compete with 0+ whitefish for the same zooplankton species, therefore the invasion of sticklebacks is unlikely to change the relative feeding pressure by individual 0+ fish on zooplankton species. The juvenile period of fish is crucial for their establishment in a new habitat as well as for the grazing impact of fish on prey communities. Here, we show that juveniles of an invasive species, sticklebacks, do have similar feeding rates on various zooplankton species compared to native whitefish despite the latter are specialized planktivores and advanced sticklebacks in growth during the seasonal course. Our results suggest that the invasion of sticklebacks and partial replacement of whitefish should not result in changing relative feeding pressures on the various zooplankton species.

Globally, our knowledge on lake fisheries is still limited despite their importance to food security and livelihoods. Here we show that fish catches can respond either positively or negatively to climate and land-use changes, by analyzing time-series data (1970–2014) for 31 lakes across five continents. We find that effects of a climate or land-use driver (e.g., air temperature) on lake environment could be relatively consistent in directions, but consequential changes in a lake-environmental factor (e.g., water temperature) could result in either increases or decreases in fish catch in a given lake. A subsequent correlation analysis indicates that reductions in fish catch was less likely to occur in response to potential climate and land-use changes if a lake is located in a region with greater access to clean water. This finding suggests that adequate investments for water-quality protection and water-use efficiency can provide additional benefits to lake fisheries and food security. Lake fisheries are vulnerable to environmental changes. Here, Kao et al. develop a Bayesian networks model to analyze time-series data from 31 major fisheries lake across five continents, showing that fish catches can respond either positively or negatively to climate and land-use changes.

The intensity and frequency of storms are projected to increase in many regions of the world because of climate change. Storms can alter environmental conditions in many ecosystems. In lakes and reservoirs, storms can reduce epilimnetic temperatures from wind-induced mixing with colder hypolimnetic waters, direct precipitation to the lake's surface, and watershed runoff. We analyzed 18 long-term and high-frequency lake datasets from 11 countries to assess the magnitude of wind-vs. rainstorm-induced changes in epilimnetic temperature. We found small day-today epilimnetic temperature decreases in response to strong wind and heavy rain during stratified conditions. Day-today epilimnetic temperature decreased, on average, by 0.28 C during the strongest windstorms (storm mean daily wind speed among lakes: 6.7 AE 2.7 m s −1 , 1 SD) and by 0.15 C after the heaviest rainstorms (storm mean daily rainfall: 21.3 AE 9.0 mm). The largest decreases in epilimnetic temperature were observed ≥2 d after sustained strong wind or heavy rain (top 5 th percentile of wind and rain events for each lake) in shallow and medium-depth lakes. The smallest decreases occurred in deep lakes. Epilimnetic temperature change from windstorms, but not rainstorms, was negatively correlated with maximum lake depth. However, even the largest storm-induced mean epilimnetic temperature decreases were typically <2 C. Day-today temperature change, in the absence of storms, often exceeded storm-induced temperature changes. Because storminduced temperature changes to lake surface waters were minimal, changes in other limnological variables (e.g., nutrient concentrations or light) from storms may have larger impacts on biological communities than temperature changes.