Hierarchical regulation of pelagic invertebrates in lakes of the northern Great Plains: a novel model for interdecadal effects of future climate change on lakes

Abstract

Endorheic lakes of the northern Great Plains encompass a wide range of environmental parameters (e.g., salinity, pH, DOC, Ca, nutrients, depth) that vary 1000-fold among sites and through the past 2000 years due to variation in basin hydrology and evaporative forcing. However, while many environmental parameters are known to individually influence zooplankton diversity and taxonomic composition, relatively little is known of the hierarchical relationships among potential controls or of how regulatory mechanisms may change in response to climate variation on diverse scales. To address these issues, we surveyed 70 lakes within a 100 000 km 2 prairie region to simulate the magnitude of environmental change expected to occur over 100-1000 years and to quantify the unique and interactive effects of diverse environmental parameters in regulating pelagic invertebrate community structure at that scale. Multivariate analyses showed that salinity was the principal correlate of changes in invertebrate composition among lakes, with a sequential loss of taxa between salinities of 4 and 50 g total dissolved solids L À1 until one to two species predominated in highly saline systems. In contrast, changes in the concentrations of Ca 2 1 and other mineral nutrients exerted secondary controls of invertebrate assemblages independent of salinity, whereas lake depth provided a tertiary regulatory mechanism structuring species composition. In contrast to these large-scale hierarchical patterns, seasonal surveys (May, July, September) of a subset of 21 lakes in each of 2003-2005 revealed that annual meteorological variation had no measurable effect on pelagic invertebrates, despite large differences in temperature, precipitation, and evaporation arising from regional droughts. Together these findings show that pelagic invertebrate communities in saline lakes are resilient to interannual variability in climate, but suggest that lakes of the northern Great Plains may provide a sensitive model to forecast centennial effects of future climate change.

Interactive effects of chemical and biological controls on food-web composition in saline prairie lakes

“…Under such circumstances, invertebrate predation often becomes important, and we did observe several large invertebrate predators (e.g., corixids, notonectids) invading the pelagic area. Yet, based on stable isotope analysis, these taxa were not part of the pelagic food web as they continued to rely on littoral diet [13]. …”

“…Hydrological connections among individual lakes exist but flow is limited to high run-off periods and unusually wet years. The 20 study lakes have been sampled for water chemistry parameters and zooplankton since 2002 [7,20], and represent similar large gradients in environmental conditions as a broader 70-lake survey that was conducted in August 2004 [7,13]. Yet in comparison to the previous study, lakes were deeper (> 3 m maximum depth), providing a well defined pelagic area that is important for a diverse fish community [23,24], and calcium concentrations remained above 10 mg L -1 , avoiding potentially confounding affects of low species diversity due to calcium limitation [25,26], [13].…”

“…Furthermore, previous work on zooplankton communities in the 20 study lakes showed that seasonal and inter-annual variability within lakes was small (< 3% of total explained variability) relative to the among lake differences [13]. Accordingly, we conducted all subsequent statistical analyses on averages over the two-year sampling period.…”

“…The decline in species richness and changes in taxonomic composition have been well described in association with increasing salinity [1,3,11,12]. Zooplankton diversity declines at higher salinities, yet, taxa occur over a very large salinity gradient with species assemblages changing from freshwater to salinity-tolerant and finally halophilic species [9,13]. In contrast, abundance and composition of littoral and benthic macroinvertebrate communities remain largely unchanged until about 20 g L -1 , beyond which diversity declines quickly ([1,14]; [15,16]).…”

“…This study is founded on a prior analysis of zooplankton composition in 70 prairie lakes [13], which identified a hierarchical control where salinity was the principal correlate of changes in taxonomic composition, while nutrients and water depth provided secondary mechanisms structuring zooplankton composition. Furthermore, a concurrent analysis of food-web complexity in these 20 lakes using stable-isotope based metrics, showed strong declines in complexity not only with salinity but also with the loss of piscivorous fishes.…”