Recovery of eelgrass (Zostera marina) after a major disturbance event in Little Egg Harbor, New Jersey, USA.

ABSTRACT: In June 1998, a macroalgal bloom occurred which rapidly caused a decline in the spatial distribution of eelgrass (Zostera marina L.). By July 1998, this bloom created a drifting mat of algal-detrital biomass in excess of 300 g ash free dry weight [m.sup.-2], which smothered and eliminated Z. marina from several locations in Little Egg Harbor, New Jersey. Subsequent to that event, frequent nuisance phytoplankton blooms (i.e., brown-tide) occurred between 1999 and 2002. During the summer of 2002, we investigated one region substantially impacted by the macroalgal loading to conduct a site assessment on the recovery of Z. marina. Spatial coverage of Z. marina and Ruppia maritima (widgeon grass) were assessed using randomized point-bused transects. While some portions of the impacted area showed post-disturbance recovery (from 0% coverage to >80% Zostera marina), a majority of the former eelgrass bed had not recovered in -four years. In fact, numerous locations contained less than 30% eelgrass coverage and some less than 10%. Results also showed that several locations had been colonized by R. maritima, suggesting that R. maritima was able to colonize the disturbed area and utilize the substrate during this time frame. This site was then revisited in 2004 and the original point-based transects were resurveyed after two years without substantial brown-tides. Results from this survey showed significant seagrass recovery with an average Z. marina spatial coverage of 97.8%. However, substantial R. maritima remained as mixed Z. marina--R. maritima beds. Ruppia maritima showed a dichotomous distribution with five stations maintaining an average spatial coverage of 44.4% mixed Z. marina--R. maritima beds and the remaining nine having an average spatial coverage of less than 1.7%. These data demonstrate that when Z. marina was eliminated from this region, R. maritima had an opportunity to colonize deeper portions of the bay where it is not generally found due to potential competitive interaction with Z. marina. However, when environmental conditions returned in favor of Z. marina growth, it rapidly revegetated the substrate.

KEY WORDS: Disturbance, Zostera marina, Ruppia maritima, eelgrass, widgeon grass.

INTRODUCTION

Seagrass communities are common in coastal tropical and temperate regions as well as portions of the sub-Arctic. They are important primary producers due to their extensive global coverage and they are a direct food resource for many grazers (Camp et al., 1973, Thayer et al., 1984, Valentine and Heck, 1991, Heck and Valentine, 1995). They also provide an indirect food resource through detrital pathways (Harrison, 1989, Vetter, 1995). Regardless of trophic pathway, seagrass biomass is an integral part of many coastal food webs and contributes to high secondary production. Additionally, numerous commercially important species (e.g., Argopecten irradians, Callinectes sapidus, Tautog onitus) use seagrasses during early development or as adult habitat. Consequently, these vegetated habitats are vital nursery grounds, often providing both food and refuge for many associated species (Heck and Thoman, 1984, Heck et al., 1997).

Seagrass structure is important in coastal regions because it dampens wave energy and reduces water velocity (Fonseca et al., 1982). The reduction of flow velocities associated with grass beds increases particle deposition (Almasi et al., 1987) and the extensive root-rhizome mat may bind particles, thereby stabilizing sediments (Thayer et al., 1984, Fonseca and Fisher, 1986). Seagrass beds, therefore, act as sediment traps and may retain finer sediments than unvegetated regions around them (Orth, 1977). The overall structure of seagrass communities covers a broad spectrum of plant species composition and aerial coverage. In general, seagrass habitats are often distributed as a mosaic of vegetated cover interspersed with varying degrees of unvegetated sediments (see Larkum and den Hartog, 1989, Robbins and Bell, 1994, Marba and Duarte, 1995). These habitat mosaics have variable shoot density, species composition, canopy height and plant biomass (Bell and Westoby, 1986, Irlandi, 1994). Therefore, seagrass habitat architecture can be defined at many spatial and temporal scales (Robbins and Bell, 1994), and defining the extent and physical arrangement of the landscape may be essential for addressing ecological questions (Holling, 1992, Levin, 1992).

It is well recognized that significant light reductions negatively impact seagrass growth and production. Additionally, it has been demonstrated that light attenuation may occur as a result of numerous factors including phytoplankton, epiphytes, and macroalgae, as well as aspects of terriginous runoff causing general turbidity. Frequently, coastal bays undergoing eutrophication exhibit high light attenuation from all of these sources (see Hauxwell et al., 2003). These supplemental stresses, while impacting the growth, production, and health of expansive beds, may greatly affect seedlings (Bintz and Nixon, 2001) and patchy or low shoot density beds (Tamaki et ...