Seagrass Density Research Articles

Seagrass patch characteristics alter direct and indirect interactions in a tritrophic estuarine food web

We used a mesocosm approach to examine howratch characteristics influenced predation and habitat selection in a tritrophic food web. Our experiments included juvenile red drum (Sciaenops ocellatus; RD), juvenile pinfish (Lagodon rhomboides; PF), and grass shrimp (Palaemonetes sp.; GS), members of a food web common in seagrass meadows of the northern Gulf of Mexico. We added an additional level of complexity to the experiment by including a predator that could feed at two different trophic levels. RD were top predators, PF were both prey items for RD and predators of GS, and GS were prey for RD and PF. We used 4 different artificial seagrass habitats that varied by size (0.049 and 0.203 m2) and shape (circular and stellate) to control for covariation between patch size and seagrass density. Predation on GS was measured in each habitat when PF, RD, and PF+RD were present, and predation on PF was measured when RD and RD+GS were present. Habitat selection by each of these 3 species was measured individually and in the presence of every other combination of the 3 species. Neither predation nor habitat selection were consistently influenced by patch characteristics (size, shape, or perimeter: area ratios) or the number of trophic levels. For GS, there was a significant negative relationship between patch size and predation rates in the GS+PF+RD treatment. Habitat selection by GS without the threat of predation suggested a preference for smaller habitats, but when in the presences of RD or RD+PF, GS preferred larger habitats. In predation experiments, PF predation by RD showed no significant relationships with patch characteristics or trophic structure. For our habitat selection experiments, PF preference was for larger habitats in the PF only and GS+PF+RD treatments. There were no significant relationships between patch size, shape, or trophic structure and RD habitat selection.

Evaluating the effect of environmental disturbance on the trophic structure of Florida Bay, U.S.A.: Multiple stable isotope analyses of contemporary and historical specimens

Widespread seagrass and sponge mortality and increased water column turbidity caused by phytoplankton blooms and suspended sediments occurred throughout Florida Bay from 1987 to the mid 1990s. This disturbance led to the hypothesis that Florida Bay was shifting from dependence on benthic production to dependence driven primarily by water-column production. We tested this hypothesis by (1) evaluating the current trophic structure of Florida Bay and then (2) comparing it to a reference site in Biscayne Bay populated by dense seagrass beds with no recent history of large-scale seagrass mortality. Additionally, we (3) compared the current trophic structure of Florida Bay to a prior trophic structure recorded in historical specimens collected before the onset of the environmental disturbance. No evidence was found to support the occurrence of a large-scale shift from a seagrass-dominated to a plankton-dominated system. The δ13C and δ34S signatures of macroinvertebrates and fishes collected from sites in Florida Bay expressed the strong dominance of benthic production and were similar to values obtained from the Biscayne Bay reference site. Highly mobile apex predators that effectively integrate the signatures of their prey over longer temporal and larger spatial scales had δ13C signatures that ranged from −13‰ to −11‰ and δ34S signatures that ranged from +2‰ to +6‰, indicating that the food web of the bay is currently dominated by strong benthic-pelagic coupling. A comparison between Florida Bay's historic communities (preserved fish from 1956–1966) and its contemporary communities also supports the conclusion that the fundamental character of the bay remains relatively unchanged.

To what extent are the characteristics of nematode assemblages in nearshore sediments on the west Australian coast related to habitat type, season and zone?

This study has determined the ways in which the characteristics of the nematode assemblages in nearshore marine waters along the microtidal lower west coast of Australia are influenced by habitat type, time of year and shore-perpendicular zones. The habitat types (1, 2 and 6), which had previously been identified on the basis of a suite of enduring environmental characteristics, could be broadly described as highly sheltered from wave activity and containing dense seagrass (1), moderately sheltered from wave activity with sparse seagrass (2) and relatively exposed to wave activity with no seagrass (6). Sampling in five consecutive seasons yielded 15 751 nematodes representing 75 species, of which, at present, only three can be referred to described species. The number of species and densities in habitat type 1, and particularly those in its subtidal zone (C), were far greater than those in the other two habitat types. Both of these biotic variables underwent marked seasonal changes, declining to low levels during winter. The compositions of the assemblages differed significantly among the three habitat types, with the differences between habitat types 1 and 6 being particularly marked. Species of Paracomesoma, Dichromadora, Marylynnia, and Pomponema, which are assumed to feed primarily on benthic diatoms, were particularly abundant at the most sheltered habitat type, whereas those of Gonionchus, Theristus and Bathylaimus, which are assumed to be deposit feeders, were relatively abundant at the most highly exposed habitat type. The compositions of the assemblages differed among seasons and were most discrete in spring, due to marked increases in the densities of certain species. However, differences in the compositions in the different zones of each habitat type were relatively small, presumably reflecting the presence of only a small tide in the region.

Seagrasses of Tongsha Island, with descriptions of four new records to Taiwan

The composition, abundance, and distribution of seagrasses on Tongsha island were studied from 21 to 29 April 2004. A total of seven seagrass species from six genera and two families were identified. Among them, four species were new to Taiwan, including Cymodocea rotundata Ehrenb. et Hempr. ex Aschers., Cymodocea serrulata (R. Br.) Aschers. and Magnus, Syringodium isoetifolium (Aschers.) Dandy, and Thalassodendron ciliatum (Forsk.) den Hartog. The species number of seagrasses was 14% of the total global number. The total coverage area of the seagrass beds was estimated to be 8.2 km^2 distributed around Tongsha island and the lagoon. Mean percentage cover and shoot density were high, ranging from 20 to 95% and ＜1250 to ＞2500 shoots m^(-2), respectively. The large area of the seagrass bed, the high shoot density and coverage of seagrasses, and the diverse species of Tongsha island indicate that the seagrass resource on Tungsha island is very abundant. These seagrass beds must be conserved to ensure the sustainable management of Tongsha Island.

The value of patches of intertidal seagrass to prawns depends on their proximity to mangroves

Penaeid prawns were sampled with a small seine net to test whether catches of postlarvae and juveniles in seagrass were affected by the distance of the seagrass (mainly Zostera capricorni) from mangroves and the density of the seagrass in a subtropical marine embayment. Sampling was replicated on the western and eastern sides of Moreton Bay, Queensland, Australia. Information on catches was combined with broad-scale spatial information on the distribution of habitats to estimate the contribution of four different categories of habitat (proximal dense seagrass, distal dense seagrass, proximal sparse seagrass, distal sparse seagrass) to the overall population of small prawns in these regions of Moreton Bay. The abundance of Penaeus plebejus and Metapenaeus bennettae was significantly and consistently greater in dense seagrass proximal to mangroves than in other types of habitat. Additionally, sparse seagrass close to mangroves supported more of these species than dense seagrass farther away, indicating that the role of spatial arrangement of habitats was more important than the effects of structural complexity alone. In contrast, the abundance of P. esculentus tended to be greatest in sparse seagrass distal from mangroves compared with the other habitats. The scaling up of the results from different seagrass types suggests that proximal seagrass beds on both sides of Moreton Bay provide by far the greatest contribution of juvenile M. bennettae and P. plebejus to the overall populations in the Bay.

Epiphytic calcium carbonate production and facies development within sub-tropical seagrass beds, Inhaca Island, Mozambique

Seagrass beds have been widely recognised as playing an important role in influencing carbonate sediment facies development. This reflects their role not only as facilitators of fine sediment settling and stabilisation, but also as substrates for epiphytic organisms that, after death, contribute skeletal carbonate to the sediment substrate. In low latitude (reef-related) settings, epiphytic carbonate production rates are often high and this, in combination with the trapping of carbonate mud produced by a range of associated calcareous algal species, typically results in the development of carbonate mud-rich facies. Whilst such environments, and their associated sediment substrates, have been widely documented, studies of seagrass facies in marginal (sub-tropical/warm temperate) marine settings have not been conducted from a sedimentological perspective. This study determines rates of epiphytic carbonate production on two seagrass species Thalassodendron ciliatum and Thalassia hemprichii, and examines seagrass sediment facies from a sub-tropical reef-related environment in southern Mozambique. Dense seagrass beds colonise primarily siliciclastic sediment substrates and are characterised by low rates of epiphytic carbonate production (mean: 43.9 g CaCO 3 m −2 year −1 for T. ciliatum, and 33.4 g CaCO 3 m −2 year −1 for T. hemprichii). Epiphytic encrusters are dominated by thin, monostromatic layers of the crustose coralline red algae Hydrolithon farinosum, along with rotaliid smaller benthic foraminifera (including Asterorotalia cf. gaimardi and Spirillina sp.) and the soritid Peneroplis sp., as well as rare encrusting acervulinid foraminifera, serpulids and bryozoans. Epiphytic calcium carbonate production rates are therefore low and this is reflected in the low (<15%) carbonate content of the seagrass sediments, as well as the low (<1%) sediment fine (<63 μm size fraction) content. This study suggests that mud-rich sediment facies do not necessarily develop in conjunction with seagrass beds and that mud-poor facies may develop in some seagrass-colonised environments. At these sites (sub-tropical and siliciclastic-sediment dominated) this is suggested to reflect both the low levels of epiphytic and benthic algal carbonate mud production as well as a net outflux of fine-grained sediment due to tidal current-induced sediment resuspension. In keeping with carbonate facies generally there are thus likely to be latitude and environment-related shifts in the composition and character of seagrass related sediment facies.

Modeling seagrass density and distribution in response to changes in turbidity stemming from bivalve filtration and seagrass sediment stabilization

In many areas of the North American mid-Atlantic coast, seagrass beds are either in decline or have disappeared due, in part, to high turbidity that reduces the light reaching the plant surface. Because of this reduction in the areal extent of seagrass beds there has been a concomitant diminishment in dampening of water movement (waves and currents) and sediment stabilization. Due to ongoing declines in stocks of suspension-feeding eastern oysters (Crassostrea virginica) in the same region, their feeding activity, which normally serves to improve water clarity, has been sharply reduced. We developed and parameterized a simple model to calculate how changes in the balance between sediment sources (wave-induced resuspension) and sinks (bivalve filtration, sedimentation within seagrass beds) regulate turbidity. Changes in turbidity were used to predict the light available for seagrass photosynthesis and the amount of carbon available for shoot growth. We parameterized this model using published observations and data collected specifically for this purpose. The model predicted that when sediments were resuspended, the presence of even quite modest levels of eastern oysters (25 g dry tissue weight m−2) distributed uniformly throughout the modeled domain, reduced suspended sediment concentrations by nearly an order of magnitude. This increased water clarity, the depth to which seagrasses were predicted to grow. Because hard clams (Mercenaria mercenaria) had a much lower weight-specific filtration rate than eastern oysters; their influence on reducing turbidity was much less than oysters. Seagrasses, once established with sufficiently high densities (>1,000 shoots m−2), damped waves, thereby reducing sediment resuspension and improving light conditions. This stabilizing effect was minor compared to the influence of uniformly distributed eastern oysters on water clarity. Our model predicted that restoration of eastern oysters has the potential to reduce turbidity in shallow estuaries, such as Chesapeake Bay, and facilitate ongoing efforts to restore seagrasses. This model included several simplifiying assumptions, including that oysters were uniformly distributed rather than aggregated into offshore reefs and that oyster feces were not resuspended.

SLIM: a numerical model to evaluate the factors controlling the evolution of intertidal mudflats in Venice Lagoon, Italy

Venice Lagoon is suffering from a deficit in sediment supply, which results in the progressive destruction of salt marshes and tidal flats [Consorzio Venezia Nuova, 1996]. Nevertheless, some of the intertidal areas are accreting showing morphological changes, which are in contrast with the general trend within the Lagoon. The morphological evolution of Palude della Centrega, a well-preserved and vegetated intertidal area located in the northern part of the Lagoon, was investigated over a period of 3 years. The short-term accretion rate was measured to be 1.52 cm/year and was used to calibrate a three-element box model, Simulation of LIttoral Morphodynamics (SLIM) constructed to predict which of the accounted factors are more relevant to the evolution of the accreting intertidal mudflat. Sensitivity analyses using SLIM on Palude della Centrega suggest that the evolution of the flats is controlled by the balance between wave erosion during Bora events and tidal sedimentation during fine weather. This balance is strongly affected by (1) turbidity of the waters flooding the tidal flats, (2) sea grass density that suppresses wave action and tidal flow and (3) biostabilisation due to microphytobenthos, which enhances stability.

Influence of Seagrasses on Water Quality in Shallow Regions of the Lower Chesapeake Bay

The influence of seagrasses on water quality was investigated seasonally from permanent stations located along transects across vegetated and formerly vegetated sites in shoal regions of the Chesapeake Bay National Estuarine Research Reserve in Virginia. The effect of the seagrass bed on conditions inside compared to outside the bed varied seasonally and could be related to bed biomass and development. During spring (April to June), the rapidly growing seagrass bed was a sink for nutrients, suspended inorganic particles, and phytoplankton, whereas during the summer, as bed dieback progressed, resuspension and release of nutrients were observed. Reductions in suspended particle concentrations and light attenuation were generally not measurable until bed biomass exceeded 50–100 gdm/m2 or 25–50% vegetative cover. During April, when nitrate levels in adjacent channel waters were observed to be highest (>10 μM), rapid uptake, equivalent to 48% of nitrogen requirements for seagrass growth, reduced inorganic nitrogen standing stocks by 73% within the bed compared to outside of it. An unvegetated site that previously supported seagrass demonstrated little capacity to reduce measurable levels of suspended particles or nutrients, and resuspension of bottom sediments contributed to higher levels of suspended particle concentrations and turbidity in the unvegetated shallows compared to adjacent waters. The capacity of seagrass beds to improve local water-quality conditions, such as turbidity and nutrients, during the spring when suspended particle concentrations are highest may be key to their continued long-term survival in this lower bay region. High levels of spring turbidity, which have been related to seagrass declines in this area, can be mediated by dense seagrass structure, but largely unvegetated areas are unlikely to modify conditions to permit survival of first-year recruits or transplants through the summer. Therefore, water-quality conditions that are suitable for recovery are likely greater than those required for continued survival of existing seagrass beds. Given this, statistically derived estimates of water-quality conditions or habitat requirements that are usually obtained from measurements in areas adjacent to existing seagrass beds should be used with caution. Although suitable for predicting the maintenance of existing beds with adequate biomass and structure, they may underestimate the levels needed for restoration and recovery of many currently unvegetated sites.

Research Article: Quality of Halodule wrightii growing near marinas

Researchers are just beginning to discover the importance of seagrasses, such as Halodule wrightii, to coastal ecosystems, but unfortunately seagrasses are being eliminated because of human manipulations. In the current study three locations of seagrass beds in Tampa Bay were studied: seagrass beds next to Tampa Bayside Marina, outside the marina, and a control group from Ft. DeSoto. The marina sites are disturbed by boating activities, which do not occur at Ft. DeSoto. It is hypothesized that the seagrasses next to the marina will have lower chlorophyll a, b, and carotenoid pigment concentrations as well as lower seagrass densities than at Ft. DeSoto because of boating disturbance and possible gasoline in the water. Although water analysis did not detect the presence of gasoline compounds in the water, seagrass density was significantly greater at Ft. DeSoto than the other two sites. Seagrass samples from each site were dissolved in Dimethylformamide to determine their photosynthetic qualities. From spectrophotometer readings at 480, 510, 646, and 664 nm the total chlorophyll a, b, and carotenoid concentrations were determined. All tests supported the hypothesis that the seagrass beds had greater amounts of chlorophyll a, b, and seagrass density, but carotenoids did not have significant difference among the three sites. This research is the first step to a project monitoring the health of H. wrightii at Tampa Bayside Marina that is being transplanted.

Experimental studies of predation by bluefish Pomatomus saltatrix in varying densities of seagrass and macroalgae

We conducted ecosystem level experiments to investigate predator-prey relationships in 3 habitat types; eelgrass Zostera marina, macroalgae, and bare sediment. Mesocosms containing marine sediments, coastal water, and varying densities of eelgrass (75 to 1000 shoots m -2 ) or macroal- gae (50 to 1000 g wet weight m -2 ) were used to simulate conditions found in shallow coastal lagoons of Rhode Island, USA. We used bluefish Pomatomus saltatrix as predators in each experiment after appropriate acclimation in each habitat type. Prey fish were provided in equally weighted rations and included silversides Menidia menidia, tautog Tautoga onitis, cunner Tautogolabrus adspersus, and menhaden Brevoortia tyrranus. Eelgrass significantly increased the survivorship of silversides, tautog and cunner at very low shoot densities. Experiments using macroalgae did not result in significantly different survival rates between bare sand and macroalgae habitat for silversides or cunner, the 2 species tested for this vegetation type. With the decline of eelgrass and increase of macroalgal cover in shallow coastal areas of Rhode Island, this information increases our understanding of the relative value of eelgrass habitat to fish stocks and should provide added incentive to protect and conserve existing eelgrass beds.

A model of the seasonal dynamics of biomass and production of the seagrass Posidonia oceanica in the Bay of Calvi (Northwestern Mediterranean)

Modelling of seagrasses can be an effective tool to assess factors regulating their growth. Growth and production model of Posidonia oceanica, the dominant submerged aquatic macrophyte occurring in the Bay of Calvi (Corsica, Ligurian Sea, Northwestern (NW) Mediterranean) was developed. The state variables are the above- and below-ground biomass of P. oceanica, the epiphyte biomass, and the internal nitrogen concentration of the whole plant. Light intensity and water temperature are the forcing variables. The model reproduces successfully seasonal growth and production for each variable at various depths (10, 20 and 30 m). The model can simulate also a number of consecutive years. Sensitivity analysis of model’s parameters showed that the maximum nitrogen quota n max rate is the most sensitive parameter in this model. The results simulations imply that light intensity is one of the most important abiotic factors, the diminution of which can cause an important reduction in seagrass density.

The influence of burrowing thalassinid shrimps on the distribution of intertidal seagrasses in Willapa Bay, Washington, USA

Two species of seagrasses frequently co-occur with extensive thalassinid shrimp populations and aquaculture operations in the intertidal zone of estuaries along the west coast of North America. Although thalassinid shrimp are known to be strong bioturbators and affect both aquaculture operations and benthic intertidal community structure, few studies have investigated shrimp–seagrass interactions. Application of the pesticide carbaryl to control shrimp populations for oyster aquaculture in Willapa Bay, Washington provided us with an experimental tool to investigate one such interaction. We found that the seagrass Zostera japonica colonized areas where ghost shrimp ( Neotrypaea californiensis) had been removed via carbaryl application. We applied carbaryl to small (900 m 2) experimental plots and compared seagrass colonization on these to that on control plots where shrimp remained abundant (100 m −2). The cumulative proportion of Z. japonica seeds and sprouts was slightly higher in the surface layer of treated plots (presumably due to the lack of shrimp bioturbation distributing them to depth), but seedling abundance was not significantly different between treated (no shrimp) and untreated control plots when they first emerged in early spring. As the season progressed however, and shrimp became more active, fewer seedlings survived in the untreated areas, and those that did survive grew much more slowly than those in the treated plots. We suspect that this was due to the effects of shrimp bioturbation and either light limitation (shoots that survived were much smaller) or direct burial and loss. Although it is an introduced plant, the natural distribution of Z. japonica is high in the intertidal zone and it is often separated from its congener Zostera marina by an expansive sandflat that is dominated by the ghost shrimp N. californiensis in west coast estuaries. The treatment of intertidal oysterbeds with carbaryl clearly reduces abundance of shrimp in this zone and we documented the same pattern of seagrass colonization on a commercial oyster bed and lack of seagrass in an adjacent unsprayed area. Density of native seagrass Z. marina shoots was also enhanced in plots treated with carbaryl, but only at lower tidal elevations or in intertidal pools where it could survive. We believe the removal of shrimp will continue to broaden the distribution of Z. japonica in Washington coastal estuaries where carbaryl use is permitted and add an interesting perspective to this controversial management issue.

Sea surface correction of high spatial resolution ikonos images to improve bottom mapping in near-shore environments

A significant number of high spatial resolution (4 m) Ikonos images acquired over shallow coastal environments present quasi-stochastic sea surface effects that seriously compromise reconnaissance of bottom features. This problem is common in wide field-of-view images where there is limited control on acquisition conditions other than cloud cover. To eliminate most of these wave and glint patterns, we use the near-infrared band, which exhibits maximum absorption and minimal water leaving radiance over clear waters, to characterize the spatial distribution of relative glint intensity, which is then scaled by absolute glint intensities in each of the visible bands. The result is subtracted from the visible bands, thus filtering out glint effects. Corrected visible bands clearly reveal seabed structural features obscured in the original data. Before- and after-correction classifications of an Ikonos image of Lee Stocking Island (Bahamas) reveal an improvement of user's accuracies for critical benthic habitat classes such as coral-dominated habitat (46.8% versus 60.5%) or dense seagrass beds (31.7% versus 52.1%). This technique offers potential to use previously discarded sections of high spatial resolution airborne or satellite images of optically shallow water for mapping substrate features.

27 Spatiotemporal distribution of abundance and density of calcareous green macroalgae along the florida keys: a synchrony study

Discerning patterns of spatiotemporal variation in wide areas is a basic step in understanding changes on coastal communities. Spatial autocorrelation or spatial synchrony (i.e. how populations covary in abundance over time) provides helpful information from population dynamics that allow us to detect common trends of variation at different spatial scales. In this study we describe the spatial and temporal variation of five calcareous green macroalgae, and analyze the synchrony of abundance and density of the five genera separately and as a functional‐group along the Florida Keys. For this study we are using data from thirty sites that have been quarterly monitored since 1996. Abundance and density of seagrass and macroalgae have been recorded using a Braun‐Blanquet methodology for the Seagrass monitoring program. We found that each genus has a distinct spatiotemporal distribution, different to the group distribution. We evaluated synchronous changes in abundance and density over time at different space scales; we found that synchrony was high between patches and lower between distinct geographical areas; and each genus showed its own synchrony pattern which was obscured when analyzed as a group. We compared the results from functional‐group to individual genus analyzing the validity of the use of functional‐groups for monitoring programs. We discuss the role of synchrony in the selection of genera for a functional‐group.

Evaluation of large-scale unsupervised classification of New Caledonia reef ecosystems using Landsat 7 ETM+ imagery

The capacity of the Landsat 7 Enhanced Thematic Mapper Plus sensor to classify the shallow benthic ecosytems of New Caledonia (South Pacific) is tested using a novel unsupervised classification method. The classes are defined by using a set of multiple spectral decision rules based on the image spectral bands. A general model is applied to the entire Southwest lagoon (5500 km 2) and tested on three representative sites: a section of the barrier reef, a cay reef flat rich in corals, and a cay reef flat rich in algae and seagrass beds. In the latter one, the classification results are compared with a locally optimized model, with aerial color photographs and extensive ground-truthed observations. Results show that a reconnaissance of the main benthic habitats in shallow areas (<5 m depth) is possible, at a geomorphological scale for coral reef structure and at a habitat scale for seagrass beds. However, results directly issued from the model must be cautiously interpreted according to empirical spatial rules, especially to avoid confusion between coral slopes and shallow dense seagrass.

Spatial variability in Sediment Oxygen Consumption under winter conditions in a lagoonal system in New Caledonia (South Pacific)

Sediment Oxygen Consumption (SOC) was investigated during a winter (Southern Hemisphere) cruise in the southwest lagoon of New Caledonia. Oxygen fluxes were measured at 11 sampling stations distributed along two coast to reef transects. Three different methods of flux measurements were used: diver-operated benthic chambers, laboratory incubation of sediment cores and oxygen microprofiles determinations. SOC values varied between 450 and 2250 μmol O 2 m −2 h −1. The level of agreement between the three techniques strongly varied as a function of sediment type. Most of the SOC values from the grey sand zone in the middle part of the lagoon and the muddy bottoms of the bays did not show significant differences. A central station presenting a dense seagrass bed gave lower SOC determined by oxygen microprofiles compared to the two other methods. In coarse carbonated sands from the back reef area, SOC measured by in situ benthic chambers were higher than SOC measured by incubation techniques. This discrepancy could be explained by physical disturbance of the sediments, macroscale variability in benthic communities or technical efficiency of the sediment sampling device and probably by a combination of all three processes. Nevertheless, for the other sediment types that represented 85% of the lagoon bottoms, the results from the three techniques used for SOC determination were strongly convergent. Based on this assumption, it could be stated that the oxygen fluxes were essentially driven by microbial activity compared to biologically mediated vertical transport of solutes. The SOC values determined during this study were in agreement with budgets previously calculated for the lagoon. Regardless of the back reef area, spatial variability in SOC can be explained by the organic matter content of sediments which clearly showed a coast to reef gradient with higher organic carbon and nitrogen contents in the coastal sediments. The C/N ratios demonstrate the higher rate of freshly deposited organic matter near the coast compared to more central stations in the lagoon.

What constitutes essential nursery habitat for a marine species? A case study of habitat form and function for queen conch

There is increasing recognition that habitats should be managed as part of fisheries management. It is generally assumed that amount of suitable habitat is linked to production of de- mersal species and that maps of bottom type will provide the information needed to conserve essen- tial habitats. In this review, a synthesis of nursery habitat is made for Strombus gigas (queen conch), a large, economically important gastropod in the Caribbean region. Juveniles occur on a variety of bottom types over their geographic range. In the Bahamas, nurseries occur in specific locations within large, beds of seagrass, while obvious characteristics of the benthic environment such as seagrass density, depth and sediment type are not good predictors of suitable habitat. Rather, nurseries persist where competent larvae are concentrated by tidal circulation and where settlement occurs selec- tively. Nursery locations provide for high juvenile growth resulting from macroalgal production not evident in maps of algal biomass, and they provide for low mortality compared with seemingly simi- lar surroundings. Therefore, critical habitats for queen conch juveniles are determined by the inter- section of habitat features and ecological processes that combine to yield high rates of recruitment and survivorship. While maps of bottom type are a good beginning for habitat management, they can be traps without good knowledge of ecological processes. A demersal species can occupy different substrata over its geographic range, different life stages often depend upon different bottom types, and specific locations can be more important than particular habitat forms. Habitat management must be designed to conserve habitat function and not just form. Implicit in the concept of 'essential habitat' is the fact that expendable habitat exists, and we need to prevent losses of working habitat because of inadequate protection, restoration or mitigation. Key nurseries may represent distinctive or even anomalous conditions.

Field evidence on the influence of seagrass landscapes on fish abundance in Bolinao, northern Philippines

On naturally occurring seagrass patches, 10 plots of 100 m 2 each were permanently marked. Seagrasses, macroalgal components and fish assemblages were monitored in February, April and June 1996. Maps of the patch configurations were produced in January 1996 by photo- graphing components and were described in terms of the area of seagrass in a plot (cover), the aver- age size of each seagrass patch, the total number of seagrass patches in a plot, number of halos, shape of a patch (perimeter:area ratio), and complexity of the edges (fractal dimension). Extraction of prin- cipal components axes (PCA) on variables describing patch and macrophyte characteristics was employed. Fish numbers from the patchy plots standardized against the fish numbers caught on the corresponding nearby continuous bed showed a positive parabolic relationship against the PCA interpreted as 'continuity of vegetation' for the months of February and June; where curvilinear regressions gave values of r = 0.85 and 0.94, respectively. In April, a negative parabola was exhibited against the PCA interpreted as 'macrophyte stands within a patch' (r = 0.73). This trend was basically due to the response of the fishes Siganus fuscescens and Abudefduf sexfasciatus dominating the catch and their preference to intermediate densities of seagrass as the optimal refuge. On the other hand, positive parabolic trends exhibited in February and June imply that fishes tended to prefer plots, which were either extremely patchy or highly continuous, while plots with spatial configura- tions of average nature were avoided. The differences in fish assemblages and/or seagrass densities within the vegetation for the sampling months explain the different responses. Management impli- cations are discussed.

Impact of hydrodynamics on development and morphology of intertidal seagrasses in the Wadden Sea

Field surveys and in situ experiments were conducted to determine the impact of hydro- dynamics (in particular tidal currents) on the development and structure of intertidal seagrass beds. Field observations in the Sylt-Romo Bight (German Wadden Sea, North Sea) from 1997 to 1999 re- vealed dense seagrass beds with a high biomass in sheltered areas, whereas at exposed sites seagrass beds occurred only sparsely with a comparatively low biomass. In addition, the shoot length and leaf length were distinctly higher at sheltered sites than in exposed beds. Cross transplantation experi- ments and enclosure experiments between sheltered and exposed seagrass sites showed that the den- sity as well as the shoot morphology (leaf number per shoot, shoot length, leaf length and leaf width) were drastically reduced after transplantation into an exposed bed, and were even lower than seagrass values of the adjacent exposed seagrasses after 6 wk. In contrast, the seagrass density as well as the length of shoots and leaves increased distinctly after transplantation into a sheltered seagrass bed. By means of an in situ 'three-current flume', experimentally modifying current flow at the same site, it was shown that increasing tidal current velocities resulted in decreased densities and length values in seagrass shoots, despite growing under the same habitat conditions. These results suggest that strong hydrodynamics directly affect the development and architecture of Zostera noltii beds by reducing seagrass density and shoot morphology, as well as the extent of seagrass beds. Since changes in envi- ronmental conditions are ongoing (e.g. higher storm frequency, sea level rise in combination with man-made protective structures, such as dikes and dams), it is suspected that increasing hydrody- namics might contribute to losses in intertidal seagrass beds.