How communities reorganize during climate change depends on the distribution of diversity within ecosystems and across landscapes. Understanding how environmental and evolutionary history constrain community resilience is critical to predicting shifts in future ecosystem function. 

The goal of our study was to understand how communities with different histories respond to environmental change with regard to shifts in elevation (temperature, nutrients) and introduced predators. We hypothesized that community responses to the environment would differ in ways consistent with local adaptation and initial trait structure. 

We transplanted plankton communities from lakes at different elevations with and without fish in the Sierra Nevada Mountains in California to mesocosms at different elevations with and without fish. We examined the relative importance of the historical and experimental environment on functional (size structure, effects on lower trophic levels), community (zooplankton composition, abundance, and biomass), and population (individual species abundance and biomass) responses.

Communities originating from different elevations produced similar biomass at each elevation despite differences in species composition; that is, the experimental elevation, but not the elevation of origin, had a strong effect on biomass. Conversely, we detected a legacy effect of predators on plankton in the fishless environment. Daphnia pulicaria that historically coexisted with fish reached greater biomass under fishless conditions than those from fishless lakes, resulting in greater zooplankton community biomass and larger average size. 

Therefore, trait variation among lake populations determined the top-down effects of fish predators. In contrast, phenotypic plasticity and local diversity were sufficient to maintain food web structure in response to changing environmental conditions associated with elevation. 

Experiment overview

We collected communities of plankton and micro-organisms from lakes that varied in their environment due to elevation, and history of fish stocking. We transplanted them to different elevations with and without fish (Figure 1). Thus, the treatments were: (1) History of elevation (HElev): source community elevation (2 levels, sub-alpine [average of 2591m] and alpine [average of 3252m]) (2) History of fish (HFish): source community fish presence (2 levels, +/-) (3) Experimental elevation treatment (EElev): transplant elevation (3 levels, montane [1200m], sub-alpine [2149m] and alpine [3093m]) and (4) Experimental fish treatment (EFish): fish presence in the transplant environment (2 levels, +/-). We used three transplant elevations (EElev) so that communities from each elevation were exposed to a lower elevation to simulate the directional environmental change these communities are predicted to experience. This results in a total of 24 treatments, each replicated five times for a total of 120 mesocosms, 40 at each experimental elevation.

Mesocosm Sampling

We sampled the mesocosms monthly for four months following the introduction of fish to quantify water chemistry, zooplankton community composition and abundance, and chlorophyll-a (a proxy for phytoplankton biomass). We sampled the zooplankton community using an integrated tube sampler. We collected 20L from haphazardly chosen locations from each mesocosm, condensed the sample on a 63μm mesh filter, and preserved it with 70% ethanol. We counted zooplankton samples using a protocol designed to estimate the abundance of common species and detect rare species: we identified two hundred individuals to the lowest taxonomic resolution possible (generally to species for crustaceans and genus for rotifers) without counting more than 50 individuals of each species toward the total. We scanned the remainder of the sample to detect rare species. While Miner et al. (B. E. Miner, Knapp, Colbourne, & Pfrender, 2013) showed that large-bodied unmelanized Daphnia “pulex-type” species is Daphnia melanica using mtDNA sequencing, we classified unmelanized “pulex-types” as D. pulicaria following (Fisk et al., 2007; Knapp & Sarnelle, 2008; Latta et al., 2007). 

To calculate zooplankton community biomass, we measured the body length of 15 individuals of the three most common species in each sample. For rare species, we used the average body length of all measured individuals of that species. Body size measurements were done only on the final set of samples (September); for these samples, we used the mesocosm-specific measurements to calculate biomass and mean body length, but for all other sample dates, we used the average length of each species across all treatments. We then used published length-weight regressions to estimate zooplankton biomass (Dumont, Van de Velda, & Dumont, 1975; McCauley, 1984). To calculate the average body size, we used an abundance-weighted mean length. Community biomass was determined by summing the population biomasses of each species.

To characterize the environment in our mesocosms, we measured a series of water chemistry variables. First, total nitrogen (TN) and total phosphorus (TP) were measured by filtering water through 63μm-mesh, collecting it in triple-rinsed 20mL high-density polyethylene (HDPE) bottles, then preserving it with H2SO4 to a pH<2 and storing it at ~4°C until later analysis. TN and TP (mg L-1) were measured using an auto-analyzer (LaChat QuikChem 8500, persulfate digestions, LaChat, Colorado, USA). We collected DOC samples by filtering water through precombusted glass fiber filters (Whatman GF/F, pore size 0.45um, Whatman, Maidstone, UK) into triple-rinsed 20mL glass vials and preserved with HCl to a pH<2. DOC (mg L-1) was measured using a total organic carbon analyzer (TOC-V CSN, Shimadzu Scientific Instruments, Kyoto, Japan). Chlorophyll-a concentration (chl-a, μg L-1), a proxy for phytoplankton biomass, was measured in a known volume of water filtered through a GF/F that we froze before processing. Chl-a concentration was measured using a Turner Trilogy fluorometer (Turner, San Jose, USA) following a 24 hour ~4°C methanol extraction. We added water (~200 L) to each mesocosm after sampling to replenish evaporated water. The evaporation rate was similar between the three locations.  

Please see the ReadMe file for units.