Carbon limitation in response to nutrient loading in an eelgrass mesocosm: influence of water residence time

Abstract

Altered primary productivity associated with eutrophication impacts not only ecosystem structure but also the biogeochemical cycling of oxygen and carbon. We conducted laboratory experiments to empirically determine how residence time (1, 3, 10 d) influences eutrophication responses in a simplified Pacific Northwest Zostera marina-green macroalgal community. We expected long-residence time (RT) systems to exhibit eutrophication impairments. Instead, we observed an accumulation of nutrients at all RTs and a shift in the dissolved inorganic carbon speciation away from CO2 (aq) with unexpected consequences for eelgrass plant condition, including shoot mortality. Most metrics responded more strongly to temperature treatments than to RT treatments. No dramatic shifts in the relative abundance of Z. marina and green macroalgae were detected. Z. marina shoot density proliferated in cool temperatures (12°C) with a modest decline at 20°C. Eelgrass loss was associated with high total scale pH (pHT) and CO2 (aq) concentrations of <10 µmol kg-1 CO2 (aq), but not with high nutrients. Z. marina d13C values support the hypothesis that carbon availability was greater at short RT. Further, very low leaf sugar concentrations are consistent with extreme photosynthetic CO2 (aq) limitation. We suggest that the effects of extremely low environmental carbon concentrations (CO2 (aq)) and increased respiration at warm temperatures (20°C) and other physiological processes can lead to internal carbon limitation and shoot mortality. Eutrophication responses to nutrient loading are more nuanced than just light limitation of eelgrass and require additional research on the interaction of the biogeochemical environment and plant physiology to better understand estuarine ecosystem disruption.