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

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 times influence eutrophication responses in a simplified Pacific Northwest Z. marina- green macroalgal community. We expected long-residence time (RT) systems to exhibit eutrophication impairments. Instead, we observed an accumulation of nutrients at all residence times and a shift in the dissolved inorganic carbon (DIC) 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 residence time treatments. No dramatic shifts in the relative abundance of Z. marina and green macroalgae were detected. Z. marina shoot density proliferated in cool temperatures with a modest decline at 20 °C. Eelgrass loss was associated with high pHT and CO2 (aq) concentrations <10 µmol kg-1 CO2 (aq) but not high nutrients. Z. marina δ13C values support the hypothesis that C 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 from warm temperatures (>20 °C) and other physiological processes can lead to internal C limitation and shoot mortality. Eutrophication responses to nutrient loading are more nuanced than just light limitation of eelgrass and requires additional research on the interaction of the biogeochemical environment and plant physiology to better understand estuarine ecosystem disruption.

Impact/Purpose

Under high nutrient conditions estuarine systems can become eutrophic causing algal blooms, low dissolved oxygen and low light conditions which result in fish kills and seagrass loss. Seagrasses are a cornerstone of healthy estuaries, providing essential fish habitat and are used as a nutrient assessment endpoint. Our results indicate that warm temperatures stimulate production more than nutrient loading. Increased photosynthesis from warm temperatures alters CO2 availability (carbonate chemistry) in the water and can lead to seagrass loss from a “negative carbon balance”. That is plants die because their metabolic activity requires more carbon than is available in the water for them to fix by photosynthesis. Further, we propose that carbon limitation was potentially a driver of seagrass death in previous studies that had been attributed solely to nitrate toxicity. We present a conceptual model that highlights multiple physiological pathways that can lead to a negative carbon balance in eelgrass and highlight how water replacement rates may influence these responses. 

Citation

Kaldy, J., C. Brown, AND S. Pacella. Carbon limitation in response to nutrient loading in an eelgrass mesocosm: influence of water residence time. Inter-Research, Luhe, GERMANY, 689:1-17, (2022). [DOI: 10.3354/meps14061]

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