Mesocosm experiments quantify the effects of eutrophication on eelgrass, Zostera marina

Abstract

Outdoor mesocosm experiments were used to examine the response of eelgrass communities to excess nutrient loading and reduced light that simulated coastal eutrophication. A series of replicated manipulations conducted between 1988 and 1990 demonstrated the effects of reduced available light and increased loading of nitrogen plus phosphorus on habitats dominated by eelgrass Zostera marina L. Shade and nutrients each significantly affected eelgrass growth, morphology, density, and biomass. WC found no significant interactions between the effects of shade and the effects of nutrients on any plant characteristics except leaf length. The growth rate of individual eelgrass shoots was linearly related to light, increasing throughout the range of available light. Biomass and daily biomass increase, or areal growth, were also linearly related to light, but specific growth showed no response to light. Shoot density increased with the log of light.Excess nutrient loading was shown to significantly reduce eelgrass growth and bed structure through stimulation of various forms of algae that effectively competed with eelgrass for light. The absence of significant interactions between the effects of shade and nutrients on eelgrass density, growth, and biomass suggests that the negative effect of algae on eelgrass occurs primarily through the reduction of light (i.e. shading). The outcome of nutrient enrichment was a shift in plant dominance from eelgrass to three algal forms: phytoplankton, epiphytic algae, and macroalgae. We quantified the effects of eutrophication and demonstrated that increased nutrient loading results in less light for eelgrass and that eclgrass growth linearly decreases with reduced light.

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

“…Several studies have demonstrated that leaf length increases with reduced light intensity but decreases with nutrient addition (e.g. Lee et al, 2007;Short et al, 1995). In our study increased nutrients did not significantly impact the leaf length of seagrasses, but this may have been due to nitrogen limiting growth (the low %N in the seagrass leaves).…”

Effect of shading by Ulva rigida canopies on growth and carbon balance of the seagrass Zostera noltii

“…Light reduction caused decreased leaf, rhizome and root elongation rates, followed by diminished production. This phenomenon has been widely recorded in previous studies, independently of the nature of the screen used (Philippart 1995, Short & Burdick 1995, Longstaff & Denninson 1999, Moore & Wetzel 2000, Havens et al 2001, Nelson & Lee 2001, Ruíz & Romero 2001, Peralta et al 2002, Brun et al 2003a. Non-structural carbohydrates were mobilized, both in above-and belowground tissues, to meet carbon demands under light limitation (Alcoverro et al 1999, Ruíz & Romero 2001, Peralta et al 2002, Brun et al 2003a, with the capacity of sucrose formation and exportation restricted in aboveground tissues, and sink strength stimulated in belowground tissues, under low light (Brun et al 2003a).…”

“…Most of the studies on the effects of light reduction on seagrass survival and growth use neutral density screens to simulate such light conditions (Philippart 1995, Short et al 1995, Lee & Dunton 1997, Moore & Wetzel 2000, Peralta et al 2002. However, the major light-absorbing components (gilvin, tripton, phytoplankton and macroalgae) in the aquatic system are not optically neutral (Kirk 1983), which becomes evident under a dense blanket of ephemeral macroalgae.…”

The primacy of top-down effects in shallow benthic ecosystems

“…This explm~ation is most often p r o p o s e d to account for the ioss of seagrasses in North kanerica (Orth and Moore 1983; N e u n d o r f e r and Kemp 1993; Short et al 1995;Tomasko et al 1996), Europe (Giesen et al 1990;den Hartog 1994), and Australia (Cambridge and M c C o m b 1984;Shepherd et al 1989). An i m p o r t a n t point is that, for the most part, these studies were conducted in the absence of consumers.…”