Untangling causal links and feedbacks among biodiversity, ecosystem functioning, and environmental factors is challenging due to their complex and context-dependent interactions (e.g., a nutrient-dependent relationship between diversity and biomass). Consequently, studies that only consider separable, unidirectional effects can produce divergent conclusions and equivocal ecological implications. To address this complexity, we use empirical dynamic modeling to assemble causal networks for 19 natural aquatic ecosystems (N24^◦~N58^◦) and quantified strengths of feedbacks among phytoplankton diversity, phytoplankton biomass, and environmental factors. Through a cross-system comparison, we identify macroecological patterns; in more diverse, oligotrophic ecosystems, biodiversity effects are more important than environmental effects (nutrients and temperature) as drivers of biomass. Furthermore, feedback strengths vary with productivity. In warm, productive systems, strong nitrate-mediated feedbacks usually prevail, whereas there are strong, phosphate-mediated feedbacks in cold, less productive systems. Our findings, based on recovered feedbacks, highlight the importance of a network view in future ecosystem management.

Varved lake sediments provide opportunities for high-resolution paleolimnological investigations that may extend monitoring surveys in order to target priority management actions under climate warming. This paper provides the synthesis of an international research program relying on >150 years-long, varved records for three managed perialpine lakes in Europe (Lakes Geneva, Annecy and Bourget). The dynamics of the dominant, local human pressures, as well as the ecological responses in the pelagic, benthic and littoral habitats were reconstructed using classical and newly developed paleo-proxies. Statistical modelling achieved the hierarchization of the drivers of their ecological trajectories. All three lakes underwent different levels of eutrophication in the first half of the XXth century, followed by re-oligotrophication. Climate warming came along with a 2°C increase in air temperature over the last century, to which lakes were unequally thermally vulnerable. Unsurprisingly, phosphorous concentration has been the dominant ecological driver over the last century. Yet, other human-influenced, local environmental drivers (fisheries management practices, river regulations) have also significantly inflected ecological trajectories. Climate change has been impacting all habitats at rates that, in some cases, exceeded those of local factors. The amplitude and ecological responses to similar climate change varied between lakes, but, at least for pelagic habitats, rather depended on the intensity of local human pressures than on the thermal effect of climate change. Deep habitats yet showed higher sensitivity to climate change but substantial influence of river flows. As a consequence, adapted local management strategies, fully integrating nutrient inputs, fisheries management and hydrological regulations, may enable mitigating the deleterious consequences of ongoing climate change on these ecosystems.

Eutrophication-like response to climate warming: an analysis of Lago Maggiore (N. Italy) zooplankton in contrasting years

In Lake Maggiore, the density of the invertebrate predator Bythotrephes longimanus increased following lake re-oligotrophication in the late 1980s. This “invasion” was followed by dramatic changes in the pelagic food web, consistent with those that followed the establishment of B. longimanus in North American lakes where it is not native. In this contribution, we explore the modern success of B. longimanus in Lake Maggiore by investigating its phenology and population density, and their correlations with abiotic and biotic factors during the period from 1981 to 2003. A 10-fold increase in the abundance of B. longimanus followed an earlier start and longer duration of annual population growth. Increased prey resources and decreased predation pressure were not observed during the B. longimanus density increase. Instead, a rise in lake temperature may have altered the reproductive cycle of this species. Furthermore, the depth and duration of a refuge from visually orienting fish predators increased during these 20 years, as a result of changes in the thermal stratification regime of the lake. This case study provides a timely example of how climatic changes may interact with biotic drivers (e.g. fish predation) to influence the density and phenology of an invertebrate predator.