Context-Dependent Eelgrass–Macroalgae Interactions Along an Estuarine Gradient in the Pacific Northwest, USA

Abstract

Land-based eutrophication is often associated with blooms of green macroalgae, resulting in negative impacts on seagrasses. The generality of this interaction has not been studied in upwelling-influenced estuaries where oceanic nutrients dominate seasonally. We conducted an observational and experimental study with Zostera marina L. and ulvoid macroalgae across an estuarine gradient in Coos Bay, Oregon. We found a gradient in mean summer macroalgal biomass from 56.1 gdw 0.25 m −2 at the marine site to 0.3 gdw 0.25 m −2 at the riverine site. Despite large macroalgal blooms at the marine site, eelgrass biomass exhibited no seasonal or interannual declines. Through experimental manipulations, we found that pulsed additions of macroalgae biomass (+4,000 mL) did not affect eelgrass in marine areas, but it had negative effects in riverine areas. In upwelling-influenced estuaries, the negative effects of macroalgal blooms are context dependent, affecting the management of seagrass habitats subject to nutrient inputs from both land and sea.

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

“…In many of these systems, marine-derived nutrients seasonally dominate estuaries, as a result of nearshore oceanographic processes rather than terrestrial nutrient loading. Here, blooms of ulvoid macroalgae can be as large as those associated with seagrass declines elsewhere, but few negative effects on the dominant seagrass species, Zostera marina L. have been detected (Thom 1990, Kentula and DeWitt 2003, Brown et al 2007, Jorgensen et al 2010, Hessing-Lewis et al 2011, Hessing-Lewis and Hacker 2013, but see Nelson and Lee 2001, Olyarnik and Stachowicz 2012. Further, in these systems, the effects of high nutrient conditions (driven by upwelling events) on eelgrass growth are unclear.…”

“…Tides in Coos Bay are mixed semidiurnal, with a mean tidal range of 2.3 m at the mouth, generating substantial tidal currents, with average flows of over 1 m/s throughout the estuary (Rumrill 2006). As a component of a corollary study (Hessing-Lewis et al 2011), background dynamics of eelgrass and macroalgae biomass were also quantified at this site from June 2007 to April 2009 (Appendix A).…”

“…Seagrasses have been particularly impacted by eutrophication both through the direct effects of nutrients and their indirect effects through macroalgal blooms (see Hemminga and Duarte 2000, Touchette and Burkholder 2000, Romero et al 2006 for reviews). However, nutrient and macroalgal effects, and their influence on seagrasses, can be context specific and dependent on local and regional circumstances (Duarte 1995, Hessing-Lewis et al 2011. Thus, an improved understanding of their relative roles, and mechanisms through which they operate, is necessary for eutrophication management across diverse coastal systems.…”

Macroalgae blooms and their effects on seagrass ecosystems

“…In a recent review, Thomsen et al, (2012) found that mass macroalgae have stronger affect on seagrasses than a small quantity of macroalgae, 'rooted' macroalgae has less effect than floating macroalgae, and bigger species of seagrass are more resistant than smaller species to macroalgae blooms. The negative effects of macroalgae blooms depend on the environmental variables in the region, impacting on the management of seagrass ecosystems subject to high nutrient loadings (Hessing-Lewis et al, 2011). Therefore, it is necessary to study the effect of macroalgae blooms on seagrass ecosystem to further understand the causes and mechanisms of seagrass decline.…”

Individual and population indicators of Zostera japonica respond quickly to experimental addition of sediment-nutrient and organic matter

“…population biomass, shoot density) could represent characteristic imprints of environmental conditions (e.g. Hauxwell et al, 2001;Hessing-Lewis et al, 2011;Martínez-Crego et al, 2008;Wood and Lavery, 2000). We did a short term manipulative experiment because small seagrasses may rapidly recover by clonal growth after modest sediment disturbance (Han et al, 2012;La Nafie et al, 2013).…”