Matthew C. Harwell

LONG‐DISTANCE DISPERSAL POTENTIAL IN A MARINE MACROPHYTE

Plant populations have long been noted to migrate faster than predicted based on their life history and seed dispersal characteristics (i.e., Reids paradox of rapid plant migration). Although precise mechanisms to account for such phenomena are not fully known for all plant species, a combination of theoretical and empirically driven mechanisms often resolves this paradox. Here, we couple a series of direct and indirect field and laboratory exercises on one marine macrophyte, Zostera marina L. (eelgrass), to measured distances between new patches and established beds in order to elucidate the long-distance dispersal and colonization potential of this marine seagrass. Detached, floating reproductive shoots with mature seeds were found to remain positively buoyant for up to 2 wk and retain mature seeds for up to 3 wk before release under laboratory conditions. Analysis of the detritus wrack along a remote shoreline found reproductive fragments with viable seeds up to 34 km from established, natural beds. A...

EELGRASS (ZOSTERA MARINA L.) SEED PROTECTION FOR FIELD EXPERIMENTS AND IMPLICATIONS FOR LARGE-SCALE RESTORATION

Abstract Eelgrass (Zostera marina L.) restoration efforts have historically focused on the use of adult vegetative shoots because of generally low success using seeds, a propagule of potential, but little-known utility, in restoration work. Previous work has shown that approximately 15% of seeds broadcast on unvegetated sediments survive to seedling stage, with losses in part resulting from predation, burial, or lateral transport. We conducted experiments using seeds in burlap bags under both laboratory and field settings to determine if protecting seeds increased survival or germination rates. Retention of seeds from preparation to initial sampling six months later was nearly 100%. Seedling survival at the field sites ranged from 41 to 56% in the burlap bag treatment, compared to 5–15% for seeds without burlap bag protection. Under laboratory conditions, seedling survival was identical in both treatments (50%). However, successful seedling growth noted in the protected treatment after 6 months was lost by 8 months because of significant sand accumulation over anchored seed bags. These preliminary results are encouraging for future restoration efforts that shift the focus to the use of seeds rather than adult plants, as greater survival of seeds in a protected environment can offer enhanced opportunities for addressing both basic and applied questions in restoration ecology.

Seed bank patterns in Chesapeake Bay eelgrass (Zostera marina L.): A bay-wide perspective

The decline of eelgrass (Zostera marina) in Chesapeake Bay in the 1960s and 1970s has been studied in the context of changes in water quality and habitat suitability; little effort has focused on the importance of reproductive ecology in understanding current and potential recovery of these populations. The spatial variability of seed-bank characteristics ofZ. marina in Chesapeake Bay was explored by a reproductive shoot and seed-bank sampling effort. Seed banks were sampled from 105 beds of submerged aquatic vegetation among 12 zones throughout the lower and middle Chesapeake Bay. Number of viable seeds was highly variable among and within zones, with seeds found in all but one zone and also found in cores not containing anyZ. marina shoots. Number of reproductive shoots was also highly variable among and within zones, with differences probably driven by different local environmental conditions. Bay-wide, viable seeds were found in more monospecificZ. marina cores than in mixed species or monospecificRuppia maritima cores suggesting local biological and environmental control on sexual reproduction. Lower densities of viable seeds in the middle Chesapeake Bay region reflect the lower abundance ofZ. marina in these regions and provide context for discussion of historical changes inZ. marina in Chesapeake Bay. While this study focused on a snap shot of the seed bank immediately after establishment, we highlight critical questions for future study that may be important for their conservation and restoration.

Biomechanical properties of the reproductive shoots of eelgrass

Abstract The biomechanical properties of freshly collected eelgrass ( Zostera marina L.) reproductive shoots from a site in the mesohaline region of the Chesapeake Bay during the period of seed release were investigated using a tensometer. Internodal segments closest to, and farthest away from the substrate were loaded in tension until they broke. Breaking stress (strength), breaking strain, toughness, and elastic modulus were calculated from the tensometer data and measurements of the final broken cross-sectional area of each internodal segment. Paired sample tests for individual shoots of the internodal segments closest to and farthest away from the substrate indicated no difference in material properties or cross-sectional area at the location of the break; however, internodal segment length was significantly longer (10%) further away from the substrate, and there is a trend of higher values (25–50% higher) for all mechanical attributes. Breaking stress, elastic modulus, and toughness, were not normally distributed, but significantly follow a Weibull distribution, while internodal segment length, shoot length, and breaking force were normally distributed. There was no relationship between strength and elastic modulus, but toughness was significantly correlated with strength, meaning strong reproductive shoots can also absorb large strain energies imparted by the environment before breaking. Mean strength, toughness, and elastic modulus are similar to other plants for which these data exist, including macroalgae. The finding of Weibull distributions in biomechanical attributes of the field population indicates that a few strong tough reproductive shoots are always present to resist extreme events, that might otherwise dislodge an entire population.

Modeling Dynamic Polygon Objects in Space and Time: A New Graph-based Technique

The analysis of dynamic spatial systems requires an explicit spatio-temporal data model and spatio-temporal analysis tools. Event-based models have been developed to analyze discrete change in continuous and feature-based spatial data. In this paper, a spatio-temporal graph model is described that supports the analysis of continuous change in feature-based polygon spatial data. The spatio-temporal graph edges, called temporal links, track changes in polygon topology through space and time. The model also introduces the concept of a spatial-interaction region that extends a models focus beyond short-term local events to encompass long-term regional events. The structure of the spatio-temporal graph is used to classify these events into five types of local polygon events and two types of spatial-interaction region events. To illustrate its utility, the model is applied to the ecological question of how patch size influences longevity in underwater plant communities in Chesapeake Bay, USA. Both a short-term local analysis and a longer-term regional analysis showed that patches of plants, or groups of patches, larger than one to two hectares in size were more likely to persist than smaller patches or groups of patches. Overall, the spatio-temporal graph model approach appears applicable to a variety of spatio-temporal questions.