Disentangling the effects of a century of eutrophication and climate warming on freshwater lake fish assemblages

Abstract

Eutrophication and climate warming are profoundly affecting fish in many freshwater lakes. Understanding the specific effects of these stressors is critical for development of effective adaptation and remediation strategies for conserving fish populations in a changing environment. Ecological niche models that incorporated the individual effects of nutrient concentration and climate were developed for 25 species of fish sampled in standard gillnet surveys from 1,577 Minnesota lakes. Lake phosphorus concentrations and climates were hindcasted to a pre-disturbance period of 1896-1925 using existing land use models and historical temperature data. Then historical fish assemblages were reconstructed using the ecological niche models. Substantial changes were noted when reconstructed fish assemblages were compared to those from the contemporary period (1981-2010). Disentangling the sometimes opposing, sometimes compounding, effects of eutrophication and climate warming was critical for understanding changes in fish assemblages. Reconstructed abundances of eutrophication-tolerant, warmwater taxa increased in prairie lakes that experienced significant eutrophication and climate warming. Eutrophication-intolerant, warmwater taxa abundance increased in forest lakes where primarily climate warming was the stressor. Coolwater fish declined in abundance in both ecoregions. Large changes in modeled abundance occurred when the effects of both climate and eutrophication operated in the same direction for some species. Conversely, the effects of climate warming and eutrophication operated in opposing directions for other species and dampened net changes in abundance. Quantifying the specific effects of climate and eutrophication will allow water resource managers to better understand how lakes have changed and provide expectations for sustainable fish assemblages in the future.

Fig 1. Niche model response surfaces.

Generalized additive model response surfaces of the joint effects of mean annual temperature (°C) and mean summer epilimnetic total phosphorus concentrations (μg/l) on the relative abundance of 25 fish species sampled in 1,577 Minnesota lakes. Relative abundance was standardized (z-score of log_e(CPE+0.1)) and rescaled by proportion of maximum fitted response.

Fig 2. Climate and lake productivity changes.

Changes in Minnesota lakes from (a) climate warming (MAT—mean annual air temperature °C) and (b) eutrophication (TP—mean summer epilimnetic total phosphorus concentrations μg/l) from pre-disturbance (1896–1925) to contemporary periods (1981–2010) for 1,236 lakes in Minnesota. Figure insets are box plots of interquartile ranges. The background in (a) represents Level 1 ecoregions [45] and (b) simplified land cover classes derived from the 2001 National Land Cover Database [44].

Fig 3. Niche space changes.

Trajectories of estimated 2-dimensional niche space changes in climate (mean annual air temperature °C) and productivity (mean summer epilimnetic total phosphorus concentration μg/l) in 1,236 Minnesota lakes from 1896 to 2010.

Fig 4. Stressor-specific changes in abundance.

Stressor-specific changes in model-estimated standardized relative abundance (z-score of log_e(CPE+0.1)) presented in units of standard deviations for 130 Minnesota lakes in the prairie, 508 lakes in the transition, and 598 lakes in the forest ecoregions from 1896 to 2010. TW = eutrophication-tolerant warmwater, TC = eutrophication-tolerant coolwater, IW = eutrophication-intolerant warmwater, and IC = eutrophication-intolerant coolwater.