Consumers mitigate heat stress and nutrient enrichment effects on eelgrass Zostera marina communities at its southern range limit

At the southern end of their range, eelgrass Zostera marina L. meadows in North Carolina, USA, are vulnerable to multiple stressors. These include eutrophication, which can promote the overgrowth of algal epiphytes, and heat stress, which causes significant seasonal die-offs during the summer. To quantify the relative influences of these ecosystem stressors on eelgrass biomass, we conducted a mesocosm experiment investigating interactive effects of heat stress (+1.5°C) and nutrient loading (+10 μM NH4NO3, 1 μM KH2PO4) on an amphipod-eelgrass system. Additionally, we manipulated consumer pressure from an abundant omnivorous fish (pinfish Lagodon rhomboides) to understand how top-down pressure interacted with bottom-up nutrient additions and warming. Pinfish consumed nearly all amphipods (which had no effect on epibiont load), prevented macroalgal accumulation, and mitigated eelgrass loss due to heat stress. Notably, when pinfish were absent from mesocosms (1) macroalgal biomass was high even though grazers were allowed to reach high densities (>600 ind. per 18 l mesocosm), and (2) warming caused an 80% reduction in eelgrass aboveground biomass. Macroalgal biomass was positively correlated with water temperature and also had a significant correlation with eelgrass aboveground biomass. Although nutrient concentrations were similar to nutrient pulses from local storm events, nutrient enrichment only increased epibiont load when combined with warming, and we ob served no discernible interactive effects on aboveground biomass. We conclude that omnivorous consumers can play an important role in preventing macroalgal accumulation from exacerbating heat stress effects, although continued warming may cause significant regional habitat loss regardless of consumer presence as eelgrass reaches its extreme physiological limits