MACROALGAL CANOPIES CONTRIBUTE TO EELGRASS (ZOSTERA MARINA) DECLINE IN TEMPERATE ESTUARINE ECOSYSTEMS

Abstract

Loss of eelgrass (Zostera marina) habitat from temperate estuaries worldwide often coincides with increased macroalgal accumulations resulting from increased delivery of anthropogenic nitrogen. We conducted macroalgal enclosure/exclosure experiments during summer 1998 within eelgrass populations in two estuaries of Waquoit Bay, Massachusetts, USA, to evaluate how increased macroalgal biomass affects density, recruitment, growth rate, and production of eelgrass. One estuary featured a low nitrogen loading rate and sustained a relatively pristine eelgrass population with a 2 cm high macroalgal canopy. The other estuary had a sixfold higher nitrogen loading rate and a declining eelgrass population with a 9 cm high macroalgal canopy. Experimental units were 1 ϫ 1 m plots of eelgrass fenced within 50 cm high plastic mesh that excluded or included macroalgae at canopy heights ranging from 0 to 25 cm. In both estuaries, rates of eelgrass loss increased, largely a result of decreased recruitment, and growth rates decreased (due to decreased rates of leaf appearance) with increasing macroalgal canopy height. Aboveground summer production in both estuaries decreased exponentially as macroalgal canopy heights increased. We conclude that macroalgal cover is a proximate cause for loss of eelgrass in the higher N estuary since, upon removal of macroalgae, we observed an increase in shoot density, a 55% increase in summer growth, and a 500% increase in summer aboveground net production. Based on summer growth data and density of shoots in our experimental plots the following spring, we suggest that the negative impacts of macroalgal canopies persist, but also that eelgrass recovery upon removal of macroalgae may be possible.To identify the mechanisms by which macroalgae potentially inhibit eelgrass production, we measured changes in nutrient and oxygen concentrations resulting from macroalgal canopies and estimated the relative importance of summer standing stocks of phytoplankton, epiphytes, and macroalgae to potential shading of eelgrass in both estuaries. We document both (1) unfavorable biogeochemical conditions (lowered redox conditions and potentially toxic concentrations of NH 4 ϩ ) imposed by the presence of macroalgal canopies and (2) potential light limitation of eelgrass by standing stocks of producers in the higher N estuary, with estimates of light reduction via macroalgae numerically more important than light sequestration by phytoplankton and epiphytes for newly recruiting shoots. Increased macroalgal biomass associated with increased nitrogen loading to estuaries can lead to eelgrass disappearance, and we identify an approximate 9-12-cm critical macroalgal canopy height at which eelgrass declines.

Combined nutrient and macroalgae loads lead to response in seagrass indicator properties

“…Increasing input of anthropogenic nutrients into coastal areas and resultant blooms of macroalgae (Burkholder et al, 2007;Hauxwell et al, 2001) may lead to seagrass ecosystem degradation. The present study demonstrated the negative effects of adding green algae U. pertusa and inorganic nutrients on Z. marina.…”

“…Seagrass decline in temperate estuaries under high nutrient enrichment often coincides with high macroalgae biomass (Burkholder et al, 2007;Hauxwell et al, 2001;Short and Burdick, 1996;Thomsen et al, 2012;Valiela et al, 1997). The negative effects of macroalgal blooms on seagrasses have been documented in many areas of the world, such as Australia, Japan, America and Europe (e.g.…”

“…The accelerated seagrass loss due to nutrient enrichment, either via direct effects or indirect effects, may eventually result in a phase shift from seagrass-towards macroalgae-dominated systems (Hauxwell et al, 2001;Montefalcone et al, 2007;Orfanidis et al, 2003). Most studies about such phase shifts from seagrasses to macroalgae have been conducted in highly eutrophic systems, where the shift to macroalgal dominance had already occurred (Cardoso et al, 2004;Montefalcone et al, 2007;Short and Burdick, 1996).…”

Macrophyte Abundance in Waquoit Bay: Effects of Land-Derived Nitrogen Loads on Seasonal and Multi-Year Biomass Patterns

“…Landderived nutrient loading to coastal waters often results in higher nutrient concentrations in estuaries (Nixon 1995;Valiela et al 1997), which stimulate primary production (Nixon 1992;Howarth 1988). This eutrophication of estuarine and coastal waters leads to increases in phytoplankton concentrations (Tomasky et al 1999), and to changes in biomass and species composition of the macroalgal canopies (Lavery et al 1991;Valiela et al 1997;Rafaelli et al 1998;Hauxwell et al 2001). Increased primary production owing to increased nutrient supply can control higher trophic levels in both benthic and pelagic food webs (Rafaelli et al 1998;Ware and Thompson 2005).…”

“…Increased primary production owing to increased nutrient supply can control higher trophic levels in both benthic and pelagic food webs (Rafaelli et al 1998;Ware and Thompson 2005). Macroalgal blooms, in particular, have had further consequences, often shading and replacing seagrass meadows (McGlathery 2001;Hauxwell et al 2001), as well as fostering of hypoxic conditions (D'Avanzo and Kremer 1994;Diaz 2001) that decrease abundance of invertebrates (Hauxwell et al 1998;Oesterling and Pihl 2001) and fish (Baden et al 1990;Deegan et al 2002).…”