The effects of lake productivity and late‐summer hypolimnetic oxygen on the size and shape of Cisco Coregonus artedi were examined in 27 Minnesota lakes. Geometric morphometry analyses of specimens captured in vertical gill nets indicated that Cisco in more productive lakes with hypoxic hypolimnia were larger, with deeper bodies, shorter snouts and caudal peduncles, and longer fins. Cisco in unproductive lakes with oxygenated hypolimnia were smaller, with slender bodies, longer snouts and caudal peduncles, and shorter fins. The effect of hypolimnetic oxygen on depths utilized by Cisco and the resulting vulnerability to predation was an important driver of shape and fin length. Deeper body depths and longer fin lengths were possibly a response to greater vulnerability to predation when hypoxic hypolimnia limit coldwater refugia for Cisco, which are forced to inhabit shallower depths near warmwater predators such as Walleye Sander vitreus and Northern Pike Esox lucius. Differences in size and shape relationships of Cisco in lakes on and off the Canadian Shield were also detected. The availability of late‐summer habitat, depth‐mediated predation risk, and growth potential appear to be important ecological drivers of morphological variation in Cisco.
Stable isotopes 13 C and 15 N are often used in lake ecosystems to assess energy sources and trophic positions, respectively. However, d 13 C and d 15 N are also influenced by internal biogeochemical processes in epilimnetic and hypolimnetic habitats in lakes, but the extent to which biogeochemical processing mediates isotope values between these two habitats, and whether these patterns are influenced by lake productivity is not known. We sampled d 13 C and d 15 N in epilimnetic mussels, Chaoborus, cisco (Coregonus artedi), and seston and zooplankton in the epilimnia and hypolimnia of 22 Minnesota (USA) lakes ranging from oligotrophic to eutrophic. We also measured lake temperature-oxygen profiles and light levels to assess factors influencing isotope patterns. Isotope samples were baseline-corrected using epilimnetic mussels in each lake (sample-mussel) to control for watershed-level differences in isotope values. Results showed d 13 C in epilimnetic and hypolimnetic zooplankton, hypolimnetic seston, Chaoborus, and cisco became more depleted in d 13 C relative to epilimnetic mussels in low-productivity lakes where light penetrated into the hypolimnion, while epilimnetic seston d 13 C stayed similar to mussel d 13 C in all lakes. This pattern was likely due to hypolimnetic phytoplankton in clearwater lakes incorporating more respired CO 2 , which is depleted in d 13 C, and subsequently passing depleted d 13 C values up the food chain. Results also showed habitat differences in d 15 N with epilimnetic and hypolimnetic zooplankton, hypolimnetic seston, Chaoborus, and cisco becoming more enriched relative to epilimnetic mussels in low-productivity lakes with higher O 2 levels in the hypolimnion, while epilimnetic seston d 15 N remained similar to mussel values. The d 15 N pattern is consistent with the idea that denitrification and microbial degradation enriched hypolimnetic seston relative to epilimnetic seston in low nutrient lakes, while enhanced epilimnetic primary production enriched epilimnetic d 15 N seston relative to hypolimnetic seston in high nutrient lakes. Our results indicate isotopic differences between epilimnetic and hypolimnetic organisms that change along productivity gradients and suggest that microbial processes and the light regime are important drivers.
Cisco (Coregonus artedi) are threatened by climate change and lake eutrophication, and their oxythermal habitat can be assessed with TDO3, the water temperature at which dissolved oxygen equals 3 mg L-1. We assessed the influence of TDO3 on cisco habitat use, genetic diversity, diets, and isotopic niche in 32 lakes ranging from oligotrophic to eutrophic. Results showed that as TDO3 increased cisco were captured higher in the water column, in a narrower band, with higher minimum temperatures and lower minimum dissolved oxygen. TDO3 was also negatively related to cisco allelic richness and expected heterozygosity, likely driven by summer kill events. Moreover, TDO3 influenced the isotopic niche of cisco, as fish captured deeper were more depleted in δ13C and more enriched in δ15N compared to epilimnetic baselines. Lastly, cisco in high TDO3 lakes consumed more Daphnia, had fewer empty stomachs, and achieved larger body size. Our work identifies specific characteristics of cisco populations that respond to climate change and eutrophication effects, and provides a framework for understanding responses of other cold-water species at the global scale.
Groundhog Day is a widespread North American ritual that marks the oncome of spring, with festivities centered around animals that humans believe have abilities to make seasonal predictions. Yet, the collective success of groundhog Marmota monax prognosticators has never been rigorously tested. Here, we propose the local climate-predicted phenology of early blooming spring plants (Carolina Spring Beauty Claytonia caroliniana, which overlaps in native range with groundhogs) as a novel and relevant descriptor of spring onset that can be applied comparatively across a broad geographical range. Of 530 unique groundhog-year predictions across 33 different locations, spring onset was correctly predicted by groundhogs exactly 50% of the time. While no singular groundhog predicted the timing of spring with any statistical significance, there were a handful of groundhogs with notable records of both successful and unsuccessful predictions: Essex Ed (Essex, CT, USA), Stonewall Jackson (Wantage, NJ, USA) and Chuckles (Manchester, CT, USA) correctly predicted spring onset over 70% of the time. By contrast, Buckeye Chuck (Marion, OH, USA), Dunkirk Dave (Dunkirk, NY, USA), and Holland Huckleberry (Holland, OH, USA) made incorrect predictions over 70% of the time. The two most widely recognized and long-tenured groundhogs in their respective countries – Wiarton Willie (CAN) and Punxsutawney Phil (USA) – had success rates of 54% and 52%, respectively, despite over 150 collective guesses. Using a novel phenological indicator of spring, this study determined, without a shadow of a doubt, that groundhog prognosticating abilities for the arrival of spring are no better than chance.
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