MARINE ECOLOGY PROGRESS SERIES Vol. 255: 127–134, 2003 Published June 24 Mar Ecol Prog Ser Mortality and productivity of eelgrass Zostera marina under conditions of experimental burial with two sediment types Katherine E. Mills1,*, Mark S. Fonseca2 1Department of Natural Resources, Cornell University, Ithaca, New York 14853, USA 2NOAA, National Ocean Service, Center for Coastal Fisheries and Habitat Research, 101 Pivers Island Road, Beaufort, North Carolina 28516, USA ABSTRACT: Mortality and productivity of Zostera marina L. were assessed to examine the effects of experimental burial using 2 types of sediment: (1) sand (6 and 0.2% silt-clay and organic matter con- tent, respectively); and (2) silt (27 and 3.3% silt-clay and organic matter content, respectively). Z. marina was buried to 0, 25, 50, 75 and 100% of its average aboveground height (16 cm) in an exist- ing eelgrass bed using 2 types of sediment characterized as either silty or sandy. Increasing percent- ages of plant burial significantly increased mortality and decreased productivity. Survival and pro- ductivity of eelgrass were substantially reduced when only 25% of the plant height was buried. Plants buried 75% or more of their height were characterized by survival and productivity measures of 0. No statistically significant differences in plant mortality or productivity were found between the 2 sediment types in this experiment. Changes in morphology of the plants were detected in measures of leaf length and surface area in a short duration (12 d) trial of the experiment, apparently in response to senescence, but etiolation was not observed. Results of this experiment indicate Z. marina can only tolerate rapid sedimentation events that cover less than half of its photosynthetic surfaces. Furthermore, the lowest levels of burial treatments (25% of plant height) resulted in mor- tality greater than 50%, indicating that even this small level of rapid sedimentation is significantly detrimental to Z. marina. KEY WORDS: Seagrass · Zostera marina · Mortality · Productivity · Burial · Deposition · Sediment type Resale or republication not permitted without written consent of the publisher INTRODUCTION librium of the system and impair the condition of sea- grass plants (McRoy & Helfferich 1980, Shepherd et al. An important function of seagrasses is their ability 1989, Vermaat et al. 1996, Duarte et al. 1997, Terrados to stabilize sediments of shallow marine areas (for a et al. 1998). In addition, vessel groundings (authors’ review see Fonseca 1996). Natural events such as pers. obs.) and the activities of burrowing animals storms can distribute sediment over plant surfaces. (Suchanek 1983, Ziebis et al. 1996, authors’ pers. obs.) However, seagrass communities typically establish a often result in substantial displacement of sediment long-term equilibrium that accommodates such events over adjacent seagrass areas. (Fonseca et al. 1983, Fonseca & Bell 1998). In addition, Burial of seagrass decreases the available photosyn- anthropogenic perturbations can increase the sedi- thetic area of the plant and increases the respiratory ment load entering seagrass beds. Increased sediment demand of the buried portion. In addition, highly loads in rivers due to upland runoff, trapping of mater- anoxic conditions are likely to occur at some depth ial by beach stabilization projects, and dredging below the sediment surface in this area (Kenworthy et and/or dumping of dredged materials offer examples al. 1982). Because rapid burial (i.e. faster than a given of anthropogenic activities that may disrupt the equi- species can alter its morphology to respond to changes *Email: [email protected] © Inter-Research 2003 · www.int-res.com 128 Mar Ecol Prog Ser 255: 127–134, 2003 in sediment level) raises the sediment surface, the a response to burial. We hypothesized that should depth of sediment anoxia almost certainly rises up- plants survive, they may show longer leaf and sheath wards toward the photosynthetic portions of the sea- lengths to a certain depth of burial, but that surface grass. These conditions may result in sulfide intrusion area would remain constant since the plants would be of the root system and other newly buried tissues from under poor conditions for increasing their total bio- the surrounding sediments (Goodman et al. 1995, Ver- mass. maat et al. 1996). This situation may inhibit the plants’ recovery from the burial event and create conditions under which the plants experience elevated physiolog- MATERIALS AND METHODS ical stress (Goodman et al. 1995), which if prolonged, could lead to death (McRoy & Helfferich 1980). How- Experimental design. Three species of seagrass are ever, due to different resource allocation strategies and found in estuarine areas near Beaufort, NC, during life history characteristics, seagrass species may vary various times of the year. Zostera marina is near the in their tolerance of sedimentation events, and some southern limit of its Atlantic range in NC. It grows species such as Amphibolis griffithii (authors’ pers. mainly between fall (October) and mid-summer (July) obs.) may be specially adapted to fluctuating sediment with its peak biomass in early June (Kenworthy et levels (Clarke & Kirkman 1989, Preen et al. 1995, al. 1982, Thayer et al. 1984). Its abundance rapidly Duarte et al. 1997, Terrados et al. 1998). declines with increasing water temperatures in August However, to our knowledge, there have been no and September. During the time of this study, Z. quantitative examinations of the tolerance of the North marina was the only seagrass species present at the American seagrasses to burial events. Therefore, to study site. Halodule wrightii and Ruppia maritima address this issue, we conducted a burial experiment begin growing in May, peak in biomass during late on one of the dominant North American species, eel- summer and decline in October (Thayer et al. 1984). grass Zostera marina L., in the North River estuary A 5 × 12 m portion of an existing, quiescent eelgrass near Beaufort, North Carolina. In this experiment, we bed was chosen in the North River near Beaufort, NC attempted to identify a threshold at which burial in- (35° 42’ N, 76° 36’ W; reported as site ‘ST1’ in previous creased the likelihood of mortality or decreased pro- studies, Murphey & Fonseca 1995, Fonseca & Bell ductivity of Z. marina. While many scenarios may be 1998), within an area of unbroken eelgrass coverage at envisioned that deposit sediment onto seagrasses, the 50 m scale. The site receives semidiurnal tides with ranging from gradual (days to weeks) to rapid (hours to an average range of approximately 0.6 to 0.8 m. Water days), we chose to rapidly bury upright plants at a temperature varied from 5.5 to 18°C and salinity mea- range of depths in an attempt to isolate the immediate sured 30 to 35 ppt during the course of the experiment. tolerance of this species to different levels of burial. The area was divided into 3 blocks, each oriented in Our study addressed changes in mortality and pro- a cross-channel direction at a constant depth. Twelve ductivity of Zostera marina caused by its burial to in- treatments were assigned to each of the 3 blocks in a creasing heights, thereby decreasing available photo- randomized complete block design. Treatment posi- synthetic surface area. We hypothesized that produc- tions were located 1 m apart within a block, with an tivity would decline with an increasing percentage of additional space of 1 m between blocks. Two sediment plant height buried due to a proportionate reduction in types, sand and silt, were obtained from areas near the the exposed leaf area able to photosynthesize. Beyond site. Sandy sediment consisted of 2.6% gravel, 91.66% some level, burial was expected to prove fatal to the sand, 5.9% silt/clay and 0.24% organic matter (OM; by plants. Organic content of the sediment, which is combustion at 500°C for 4 h). The silt sediment was strongly correlated with particle size in our systems, composed of 0% gravel, 73.3% sand, 26.7% silt/clay contributes to a higher sulfide content which may fur- and 3.31% OM. ther stress the plants (Kenworthy et al. 1982, Carlson An arbitrary sample of plants in the study area indi- et al. 1994, Goodman et al. 1995). Therefore, to test the cated that the average height (sheath plus blades) of effect of varying levels of sediment composition on Zostera marina at the time of this study was approxi- plant mortality and productivity, we buried plants in mately 16 cm. Five burial treatments using each sedi- both silt and sand sediment types, where silty sedi- ment type were established based on this average ments had naturally higher organic content. We ex- overall height. Plants were buried to 0% (0 cm), 25% pected lower productivity and higher mortality for (4 cm), 50% (8 cm), 75% (12 cm) or 100% (16 cm) of plants buried in silt sediment than those buried in the average height. Burial treatments were contained sand. Finally, we anticipated the possibility of morpho- within 20 cm tall by 15.2 cm in diameter polyvinylchlo- logical changes in the plants themselves caused either ride (PVC) cylinders (Fig. 1). Cylinders were placed at by etiolation (Marbà & Duarte 1994) or deterioration as a depth of ~4 cm into the substrate, approximately to Mills & Fonseca: Burial of Zostera marina 129 the depth of the plant rhizomes. Care was taken not to used for the second trial (March 31 to April 13, 1995). sever the rhizomes; however, we cannot ensure that Although productivity could not be assessed during the rhizomes were not pushed slightly deeper into the the first trial (24 to 28 d), mortality calculations were sediment in this process. Cylinders stood aboveground successfully obtained. The second, shorter trial (12 d) to a height of 16 cm and this portion remained fully was used to determine productivity differences among submersed at low tide.