Eelgrass-macroalgae interactions; context-dependency in upwelling-influenced estuaries

Graduation date: 2012This dissertation investigates the context-dependency of species interactions between seagrass and macroalgae in upwelling-influenced estuaries. In all coastal systems, nutrient loading is multidirectional, resulting from mostly freshwater and marine inputs. The directionality of nutrient inputs may affect the rate of supply of organic matter to the system. In systems where freshwater nutrient loading dominates, and has increased through time, research shows that blooms of fast-growing macroalgae often result in loss of critical seagrass habitats. In upwelling-influenced systems, marine- based nutrient inputs dominate during the summer, also resulting in blooms of ulvoid macroalgae during these productive months. The dominance of marine nutrients in these estuaries, coupled with additional variation in the physicochemical characteristics of seagrass beds, present novel contexts to study the outcomes of species interactions between the seagrass (Zostera marina L. (eelgrass) and ulvoid macroalgae. I studied these interactions at two different spatial scales that both address the relative importance of marine versus terrestrial nutrient sources on interaction\ud outcome. Regionally, I studied between-estuary, latitudinal patterns in species interactions relative to differences in marine and terrestrial drivers of nutrient loading. Within an estuary, I also compared interactions among zones along an estuarine gradient, where nutrient patterns were reflective of the relative contribution of marine- based nutrients.\ud At both scales of inquiry I employed both observational and experimental approaches to quantify species interaction dynamics. At the regional scale, I used a 5- year observational dataset from four estuaries along the Oregon and Washington coasts to study the relationship between eelgrass and ulvoid macroalgae (Chapter 2). Across latitudes that span ~220 km, macroalgal production was highest in the southern estuaries, and associated with decreased eelgrass production compared to the northern estuaries. However, through time, no estuarine site, regardless of its macroalgal biomass, was associated with declining eelgrass biomass. Contrary to systems where macroalgal production is driven by terrestrial inputs, I found that blooms in upwelling- influenced systems were associated with both marine and terrestrial drivers of nutrient inputs and production.\ud In Coos Bay (South Slough), at the within-estuary scale, I also found differences in macroalgal and eelgrass biomass among sites along an estuarine gradient. Here too, based on a 2-year seasonal dataset of producer dynamics, I found no temporal relationship between eelgrass and macroalgae producer dynamics (Chapter 3). I used a comparative-experimental framework to understand the impact of macroalgal manipulations (additions and removals) on interactions with eelgrass along\ud this gradient. In intertidal seagrass beds in the marine and polyhaline zones of the estuary I found that interaction strength was neutral and sometimes positive. However, in the riverine zone, interaction strength was negative, caused by decreased eelgrass density following macroalgal manipulation.\ud To further examine the mechanisms informing interaction outcomes in the marine zone, a large-scale macroalgal manipulation was conducted, coupled with a mesocosm experiment (Chapter 4). For the mesocosm experiment I manipulated macroalgae and nutrients as in the field, but found dissimilar results. In the mesocosms, where water movement was limited and no tidal action occurred, negative effects of macroalgal addition were found. These were associated with increased light attenuation and decreased sediment oxygen levels. Contrary to these results, I found no macroalgal, or covariate effects in the field experiment. I also manipulated water column nutrients in both experiments, and found limited effects of nutrient enrichment on eelgrass, but not macroalgae, in the mesocosm experiment.\ud Throughout these studies I demonstrated that the mechanisms determining context-dependency in upwelling-influenced estuaries are informed by physical and biogeochemical conditions, coupled with high ambient marine-derived nutrient concentrations. These findings are important to coastal management because they suggest that the strength, direction and mechanisms of interactions are shaped by local abiotic conditions and long-term nutrient regimes, rather than high nutrient concentrations per se. Given the shifting nature of nutrient concentrations in coastal waters associated with both coastal development and climate change, knowledge of\ud context dependency can also be used to assess and forecast future changes in species interactions