Are large macroalgal blooms necessarily bad? nutrient impacts on seagrass in upwelling‐influenced estuaries

Abstract

Knowledge of nutrient pathways and their resulting ecological interactions can alleviate numerous environmental problems associated with nutrient increases in both natural and managed systems. Although not unique, coastal systems are particularly prone to complex ecological interactions resulting from nutrient inputs from both the land and sea. Nutrient inputs to coastal systems often spur ulvoid macroalgal blooms, with negative consequences for seagrasses, primarily through shading, as well as through changes in local biogeochemistry. We conducted complementary field and mesocosm experiments in an upwelling-influenced estuary, where marine-derived nutrients dominate, to understand the direct and indirect effects of nutrients on the macroalgal-eelgrass (Zostera marina L.) interaction. In the field experiment, we found weak evidence that nutrients and/or macroalgal treatments had a negative effect on eelgrass. However, in the mesocosm experiment, we found that a combination of nutrient and macroalgal treatments led to strongly negative eelgrass responses, primarily via indirect effects associated with macroalgal additions. Together, increased total light attenuation and decreased sediment oxygen levels were associated with larger effects on eelgrass than shading alone, which was evaluated using mimic algae treatments that did not alter sediment redox potential. Nutrient addition in the mesocosms directly affected seagrass density; biomass, and morphology, but not as strongly as macroalgae. We hypothesize that the contrary results from these parallel experiments are a consequence of differences in the hydrodynamics between field and mesocosm settings. We suggest that the high rates of water movement and tidal submersion of our intertidal field experiments alleviated the light reduction and negative biogeochemical changes in the sediment associated with macroalgal canopies, as well as the nutrient effects observed in the mesocosm experiments. Furthermore, adaptation of ulvoids and eelgrass to high, but variable, background nutrient concentrations in upwelling-influenced estuaries may partly explain the venue-specific results reported here. In order to manage critical seagrass habitats, nutrient criteria and macroalgal indicators must consider variability in marine-based nutrient delivery and local physical conditions among estuaries.

Macrophyte community response to nitrogen loading and thermal stressors in rapidly flushed mesocosm systems

“…However, the lack of a direct NO 3 toxicity effect in this experiment is consistent with previous work where plants exposed to 1000 μM NO 3 or NH 4 exhibited no detectable stress or mortality (Kaldy 2012). Likewise, Hessing-Lewis et al (2015) exposed eelgrass plants to very high nutrient loads and found little evidence for nutrient effects. Some seagrass populations may be more susceptible to thermal or other stressors than to direct nutrient stressors.…”

Evidence for regional‐scale controls on eelgrass (Zostera marina) and mesograzer community structure in upwelling‐influenced estuaries

“…What are some of the possible explanations for the preponderance of regional‐scale patterns in eelgrass community structure in these estuaries? Considering primary producers first, we know from previous observational and experimental work in these estuaries that eelgrass and macroalgal abundances are largely influenced by ocean upwelling rather than competition with other primary producers (Hessing‐Lewis et al , ; Hessing‐Lewis and Hacker ). We found similar patterns in this study; the two estuaries with greatest upwelling intensity (i.e., YB and CB) had much lower eelgrass shoot densities and biomass than the estuary with less upwelling exposure (NB; Fig.…”

The good, the bad and the Ulva: the density dependent role of macroalgal subsidies in influencing diversity and trophic structure of an estuarine community

“…These pressed subsidies affect biogeochemical processes that alter habitat suitability for benthic fauna, may change over different temporal scales (Nedergaard et al ) and thus may explain temporal variability in the effects of macroalgal subsidies on benthic communities (Raffaelli ). Pressed macroalgal subsidies may alter abiotic conditions by restricting light (Olyarnik and Stachowicz ) and oxygen availability (Hessing‐Lewis et al ). Furthermore, hypoxic and anoxic conditions caused by pressed macroalgal mats alter nutrient cycling (Havens et al ) and, coupled with the release of organic matter during macroalgal decomposition, result in elevated concentrations of hydrogen sulfide (Hargrave et al ).…”