Jacques van Montfrans

Faunal communities in seagrass beds : a review of the influence of plant structure and prey characteristics on predator-prey relationships

When compared with nearby unvergetated areas, seagrass meadows contain a dense and strikingly rich assemblage of vertebrates and invertebrates. Most recent literature has focused on evaluating the role of predation in structuring seagrass faunal communities; however, habitat complexity, abundance of food and sediment stability may also be important. This paper summarizes studies relating predator-prey relationships to different features of the seagrass system. This review suggests that the abundance of many species, both epifauna and infauna, is positively correlated with two distinct aspects of plant morphology: 1) the root-rhizome mat, and 2) the plant canopy. A scheme was developed that defines the conditions under which any particular species will be abundant or rare in a seagrass assemblage. This scheme is based on prey and predator characteristics (e.g., epifaunal vs. infaunal, tube-dweller vs. nontube dweller, burrowers vs. nonburrowers, and large vs. small as adult) and on characteristics of the seagrasses (e.g., leaf morphology, shoot density, shoot biomass, structural complexity of the meadow, and root-rhizome density and standing crop).

Epiphyte-seagrass relationships with an emphasis on the role of micrograzing: A review

Abstract Despite the recent advances in seagrass ecology over the last ten years, there are still numerous aspects of the ecological and biological interactions that occur in seagrass ecosystems that remain poorly understood. We have attempted, here, to place into perspective one interrelationship that could have important implications in the production and vigor of seagrasses. This is the relationship between epiphytic fouling by macroalgae and periphyton and the grazers which consume them as a food source while leaving the leaves intact. Our approach to this review was first to describe the relationships between macroalgae, periphyton and the seagrass host in terms of physical benefits, biochemical interactions, factors which reduce fouling on the host, and the effects of epiphytism on seagrass photosynthesis. We then examined the importance of epiphytes as a food source for those herbivores found in seagrass beds, and looked at the consequences of this grazing and removal of epiphytes for the seagrass host. Based on the potential impact of epiphytes on seagrass and grazers on epiphytes, we developed a hypothetical model that describes the effect of increasing epiphytic fouling on seagrass production in the presence and absence of grazers. From this model, we have made predictions on the direction of seagrass decline with diminishing light along depth and estuarine gradients. Lastly, we touched briefly on the problem of eutrophication and how it affects the balance of these interrelationships, and the management options to insure the health and survival of seagrass habitats in the face of increasing stress by man on these critically important ecosystems.

Epiphyte-grazer relationships in seagrass meadows: consequences for seagrass growth and production

Studies of seagrass meadows have shown that the production of algal epiphytes attached to seagrass blades approaches 20% of the seagrass production and that epiphytes are more important as food for associated fauna than are the more refractory seagrass blades. Since epiphytes may compete with seagrasses for light and water column nutrients, excessive epiphytic fouling could have serious consequences for seagrass growth. We summarize much of the literature on epiphytegrazer relationships in seagrass meadows within the context of seagrass growth and production. We also provide insights from mathematical modeling simulations of these relationships for a Chesapeake BayZostera marina meadow. Finally we focus on future research needs for more completely understanding the influences that epiphyte grazers have on seagrass production.

Density-Dependent Settler-Recruit-Juvenile Relationships in Blue Crabs

Current theory on the population dynamics of marine species with complex life history patterns posits that a suite of physical and biotic forces (e.g., habitat structure and density-dependent predation or emigration) control survival and abundance in early life history, particularly after settlement. We have conducted a long-term sampling effort accompanied by a series of field and laboratory experiments examining the joint effects of habitat type, body size, and population density upon abundance and survival of early juveniles of the blue crab, Callinectes sapidus. In addition, the chance occurrence of a tropical storm during one set of experiments provided an opportunity to assess the impact of a physical disturbance upon newly settled blue crab survival and abundance. In the 10- yr sampling effort, we quantified relationships between sequential life history stages (ju- venile crab instars) in seagrass beds, the initial nursery habitat for blue crabs in the lower Chesapeake Bay. Inter-instar relationships were defined as the densities of larger instars as dependent on the densities of smaller instars. Inter-instar relationships for the youngest instars are described by hyperbolic functions-until crabs begin to emigrate to unvegetated habitats at approximately the fifth instar. Inter-instar relationships between crabs larger than the fifth instar and smaller crabs become either parabolic or linear functions and decay as the number of instars between sequential life history stages increases. While both the hyperbolic and parabolic functions are indicative of populations regulated by density-de- pendent processes, either predation or emigration, the decay in the functions describing the inter-instar relationships for crabs larger than the fifth instar indicates that the suite of processes regulating this segment of the population changes qualitatively. In laboratory and field experiments, the effects of vegetated and unvegetated habitats and size-specific predation on newly settled juveniles were tested. Tethering was used to quantify relative rates of predation, and a laboratory study was conducted to determine if tethering induced treatment-specific bias. We found no statistically significant interactions between the tethering treatment and the factor treatments of crab size and habitat during the laboratory study, indicating that tethering did not produce treatment-specific bias. Thus, tethering provided a relative measure of predation that allowed comparisons between treat- ments of habitat and crab size on crab survival. In both laboratory and field experiments, survival was significantly higher in vegetated habitats and with increasing size until the ninth instar, when survival did not differ by habitat. This difference explains the dispersal from vegetated to unvegetated habitats that occurred between the fifth and seventh instars. In addition, survival of all crabs was significantly increased both during and after Tropical Storm Danielle compared to pre-storm conditions. A model is developed that describes juvenile survival as a function of crab size and habitat type. Survival curves in both habitats are represented by similar sigmoid functions with survival higher in vegetated habitats. Subsequently, the survival of newly settled blue crabs is likely dependent on the availability of complex habitat. Thus, a suite of biotic and physical processes, both density-dependent and density-independent, control the early life history after settlement for the blue crab.

Preliminary studies of grazing by Bittium varium on eelgrass periphyton

Abstract The grazing activities of Bittium varium Pfeiffer on periphyton colonizing live eelgrass ( Zostera marina L.) and artificial eelgrass (polypropylene ribbon) were investigated. Quantitative measurements of grazing impact on artificial substrates were determined by periphyton pigment extraction and dry weight differences between grazed and ungrazed blades. Significant differences occurred in phaeophytin and dry weight calculations, but chlorophyll a measurements were not significantly different. This suggests that senescent diatoms constituted the bulk of the periphyton weight and were selectively removed over more actively photosynthesizing diatoms. An examination of scanning electron micrographs further elucidated the impact of grazing by Bittium varium . Some micrographs revealed that B. varium removed primarily the upper layer of the periphyton crust on both artificial substrates and living Zostera marina . The diatom Cocconeis scutellum Ehrenb. which attaches firmly to living Z. marina blades was less commonly removed than Nitzchia or Amphora . Through its grazing activities, B. varium may maintain community dominance by tightly adhering diatoms such as C. scutellum . Evidence of the complete removal of periphyton exposing the Z. marina epithelium was revealed in other micrographs. The grazing activities of Bittium varium , which removes periphyton from seagrass blades, could have important implications for the distribution and abundance of Zostera marina in the Chesapeake Bay.

Blue crab megalopal influx to Chesapeake Bay: Evidence for a wind-driven mechanism

Abstract Field surveys indicate that blue crab larvae and postlarvae develop in shelf waters adjacent to the Chesapeake Bay entrance, and that postlarvae return to the estuary for settlement into nursery areas. The postlarval form is the megalopa, and in the offshore area most of these are found near the surface. However, the surface mean flow at the Bay entrance is seaward. Megalopae must either drop to the bottom to become entrained in the density-driven inflow or employ another transport process in the surface. A potentially important mechanism by which these megalopae can return is through episonic wind-driven exchange, which is a prominent feature of the circulation in this region. Using sea level data, the magnitude of the wind-induced changes in Bay volume can be calculated for any period when these data are available. During 1985–1987, megalopae were collected daily in the York River (a tributary of Chesapeake Bay) from August through November. Their temporal distribution was characterized by pulses of individuals, separated by periods when very few were collected. A total of 12 of 16 observed megalopal pulses occurred during positive volume anomalies. In particular, the largest peak of 1985 occurred during the massive storm surge associated with Hurricane Juan, implying large-scale transport of megalopae from the shelf. Analysis of 28 years of subtidal volume data indicates that an average of 10 major inflow events per year occur during the period when megalopae are present. This indicates that these wind-induced inflow events are not fortuitous but rather are a stable feature of the flow climate at the Bay entrance.

Can otolith chemistry be used for identifying essential seagrass habitats for juvenile spotted seatrout, Cynoscion nebulosus, in Chesapeake Bay?

We investigated the variability of otolith chemistry in juvenile spotted seatrout from Chesapeake Bay seagrass habitats in 1998 and 2001, to assess whether otolith elemental and isotopic composition could be used to identify the most essential seagrass habitats for those juvenile fish. Otolith chemistry (Ca, Mn, Sr, Ba, and La; δ13C, δ18O) of juvenile fish collected in the five major seagrass habitats (Potomac, Rappahannock, York, Island, and Pocomoke Sound) showed significant variability within and between years. Although the ability of trace elements to allocate individual fish may vary between years, in combination with stable isotopes, they achieve high classification accuracy averaging 80–82% in the Pocomoke Sound and the Island, and 95–100% in the York and the Potomac habitats. The trace elements (Mn, Ba, and La) provided the best discrimination in 2001, a year of lower freshwater discharge than 1998. This is the first application of a rare earth element measured in otoliths (La) to discriminate habitats, and identify seagrass habitats for juvenile spotted seatrout at spatial scales of 15 km. Such fine spatial scale discrimination of habitats has not been previously achieved in estuaries and will distinguish fish born in individual seagrass beds in the Bay.

Population dynamics of blue crabs Callinectes sapidus Rathbun in a lower Chesapeake Bay tidal marsh creek

Abstract The dynamics of a blue crab Callinectes sapidus Rathbun population residing in an ≈ 10 000-m 2 tidal marsh creek, lower Chesapeake Bay, Virginia, were examined. During the 65-day, single-mark, multiple-recapture study, crabs were marked with an internal coded wire tag injected into the left or right basal muscle of the 5th pereiopod. Estimates of population size in the creek ranged from 799 to 1564 individuals throughout the study, yielding density estimates of 0.08–0.15 ind·m −2 . The population was dominated by medium-sized (50–100 mm carapace width) individuals. Loss of crabs (emigration plus mortality) was calculated from tag-recovery data, and immigration was determined as the difference between the change in population size and the loss of individuals. Crab movement into and out of the creek was not tidally driven. Median residency (tag half life) was 8–12 days, with population turnover exceeding 65 days. Residency estimates were used as an indication of potential marsh production exchange with other adiacent habitats. On short time scales (i.e., tidally or daily), blue crab fecal deposition may not be an important mechanism for transferring marsh production to adjacent estuarine habitats. Instead, incorporation of marsh production into body tissues (i.e., growth) followed by emigration over longer time scales (i.e., weeks to months) appears to play a greater role in coupling marsh production with adjacent habitats.

Fish Species Distribution in Seagrass Habitats of Chesapeake Bay are Structured by Abiotic and Biotic Factors

Abstract Seagrass habitats have long been known to serve as nursery habitats for juvenile fish by providing refuges from predation and areas of high forage abundance. However, comparatively less is known about other factors structuring fish communities that make extensive use of seagrass as nursery habitat. We examined both physical and biological factors that may structure the juvenile seagrass-associated fish communities across a synoptic-scale multiyear study in lower Chesapeake Bay. Across 3 years of sampling, we collected 21,153 fish from 31 species. Silver Perch Bairdiella chrysoura made up over 86% of all individuals collected. Nine additional species made up at least 1% of the fish community in the bay but were at very different abundances than historical estimates of the fish community from the early 1980s. Eight species, including Silver Perch, showed a relationship with measured gradients of temperature or salinity and Spot Leiostomus xanthurus showed a negative relationship with the presence of macroalgae. Climate change, particularly increased precipitation and runoff from frequent and intense events, has the potential to alter fish—habitat relationships in seagrass beds and other habitats and may have already altered the fish community composition. Comparisons of fish species to historical data from the 1970s, our data, and recent contemporary data in the late 2000s suggests this has occurred.