Shallow Subtidal Sites Research Articles

Resilience and Species Accumulation across Seafloor Habitat Transitions in a Northern New Zealand Harbour

Biodiversity is crucial for maintaining ecosystem stability and functionality under increasing anthropogenic stress. Part of this resilience comes from having many species performing the same function (functional redundancy) leading to the quantification of community composition and functional redundancy in relation to increasing stress. However, much of the research within coastal ecosystems focuses on distinct areas, rather than whole ecosystems. Here, we investigate the relationship between biodiversity and functional redundancy across two environmental gradients (sediment mud content and water column depth) and different habitat types following a survey of benthic macrofauna and sediment characteristics at 24 sites within Whangārei Harbour, New Zealand. We observed strong gradients in biodiversity which fragmented communities into fewer species that were a subset of the wider community. The lowest biodiversity was observed at muddy, intertidal and shallow subtidal sites which also had the lowest predicted functional redundancy. We show the stronger influence of water column depth on predicted functional redundancy than sediment mud content, highlighting the importance of subtidal regions. Overall, our study highlights the importance of studying the individual contributions of different areas in a landscape to characterise effective colonist pool size and how this can be used to predict recovery potential following disturbance.

Dominance of the scleractinian coral Alveopora japonica in the barren subtidal hard bottom of high-latitude Jeju Island off the south coast of Korea assessed by high-resolution underwater images.

Coastal benthic communities in temperate regions have been influenced by climate change, including increasing sea-surface temperature. Nevertheless, scleractinian coral Alveopora japonica Eguchi, 1968, is thriving in shallow subtidal hard bottoms around Jeju Island, off the southern coast of Korea. The presence of this corals has negatively impacted subtidal kelp populations in Jeju Island. However, there is no study to document how the presence or absence of this coral relates to other benthic communities. This study investigated the benthos in three shallow subtidal sites (Shinheung (SH), Bukchon (BC), and Seongsan (SS)) in northern Jeju using underwater photography. Macro-benthic organisms appearing on a 1 × 20 m line transect installed at depths of 5, 10, and 15 m at each site were analyzed. Results showed that of the three sites investigated, A. japonica colonies were most abundant at BC, accounting for 45.9% and 72.8% of the total transect area at 10 m and 15 m, respectively. At SS, A. japonica occupied 15.3% of the total area at 15 m and less than 1% at 5 m and 10 m. The same at SH accounted for 10% of the total area at 5 m, and less than 1% at 10 m and 15 m. Dead and bleached colonies accounted for 1.2-11.5% and 1.8-5.7%, respectively, at 5, 10, and 15 m at three sites. At SS, canopy-forming brown algae Ecklonia cava and Sargassum spp. accounted for 20.2 and 24.3% of the total transect area, respectively, at 5 m depth. In contrast, the percent cover of E. cava and Sargassum spp. at SH and BC ranged from 0.1 to 1.8%, respectively. Moreover, non-geniculate coralline algae dominated the subtidal substrate at SH, ranging between 60.2 and 69% at 15 and 10 m. The low cover of A. japonica in SS (at 5 m) coincided with a high percent cover of canopy-forming brown algae. However, canopy-forming brown algae were rare at all depths at SH and BC and were dominated instead by coralline algae and the scleractinian corals. This study, by utilizing a non-destructive method, provides a baseline qualitative and quantitative information for understanding the site and depth-dependent distribution of A. japonica and algal populations, which is important to understand climate change related changes in benthic communities in Jeju and elsewhere.

Microbial river-to-sea continuum: gradients in benthic and planktonic diversity, osmoregulation and nutrient cycling

BackgroundCoastal aquatic ecosystems include chemically distinct, but highly interconnected environments. Across a freshwater-to-marine transect, aquatic communities are exposed to large variations in salinity and nutrient availability as tidal cycles create periodic fluctuations in local conditions. These factors are predicted to strongly influence the resident microbial community structure and functioning, and alter the structure of aquatic food webs and biogeochemical cycles. Nevertheless, little is known about the spatial distribution of metabolic properties across salinity gradients, and no study has simultaneously surveyed the sediment and water environments. Here, we determined patterns and drivers of benthic and planktonic prokaryotic and microeukaryotic community assembly across a river and tidal lagoon system by collecting sediments and planktonic biomass at nine shallow subtidal sites in the summer. Genomic and transcriptomic analyses, alongside a suite of complementary geochemical data, were used to determine patterns in the distribution of taxa, mechanisms of salt tolerance, and nutrient cycling.ResultsTaxonomic and metabolic profiles related to salt tolerance and nutrient cycling of the aquatic microbiome were found to decrease in similarity with increasing salinity, and distinct trends in diversity were observed between the water column and sediment. Non-saline and saline communities adopted divergent strategies for osmoregulation, with an increase in osmoregulation-related transcript expression as salinity increased in the water column due to lineage-specific adaptations to salt tolerance. Results indicated a transition from phosphate limitation in freshwater habitats to nutrient-rich conditions in the brackish zone, where distinct carbon, nitrogen and sulfur cycling processes dominated. Phosphorus acquisition-related activity was highest in the freshwater zone, along with dissimilatory nitrate reduction to ammonium in freshwater sediment. Activity associated with denitrification, sulfur metabolism and photosynthesis were instead highest in the brackish zone, where photosynthesis was dominated by distinct microeukaryotes in water (Cryptophyta) and sediment (diatoms). Despite microeukaryotes and archaea being rare relative to bacteria, results indicate that they contributed more to photosynthesis and ammonia oxidation, respectively.ConclusionsOur study demonstrates clear freshwater–saline and sediment–water ecosystem boundaries in an interconnected coastal aquatic system and provides a framework for understanding the relative importance of salinity, planktonic-versus-benthic habitats and nutrient availability in shaping aquatic microbial metabolic processes, particularly in tidal lagoon systems.72ugw57JLuZNUPngo1htqPVideo abstract

A novel bycatch reduction device (BRD) and its use in a directed fishery for non-indigenous green crabs (C. maenas) in Atlantic Canada

The European green crab (Carcinus maenas) is one of the world’s most successful invasive species. Even though most attempts at control of this species have failed, a recently implemented fishery in Atlantic Canada aims to control its populations and mitigate its negative effects. The use of fyke nets to capture green crabs could reduce the cost of fishing and increase the financial incentive to harvest this species. However, these nets are currently not permitted for fishing this species because they potentially capture large amounts of bycatch in a sensitive ecological zone. Hence, the primary goal of this study was to design and assess the effectiveness of a novel bycatch reduction device (BRD) installed on traditional fyke nets. Fyke nets were deployed in pairs at four shallow subtidal sites in eastern Prince Edward Island and sampled over 16 consecutive days in summer 2016. Both catch (green crab) and bycatch counts in the original fyke nets were significantly higher than in the nets equipped with a BRD. However, bycatch diversity was significantly lower in the BRD equipped nets, and proportionally less detrimental to three economically and ecologically important bycatch species.

Impact of electrogenic sulfur oxidation on trace metal cycling in a coastal sediment

Electrogenic sulfur oxidation (e-SOx) is a newly discovered pathway of microbial sulfide oxidation, mediated by so-called cable bacteria. The reduction of oxygen near the sediment-water interface is coupled by long-distance electron transport to the oxidation of sulfide in deeper sediment, and in this way, electrical currents are induced within the seafloor that range over centimeter scale distances. Previously, electrogenic sulfur oxidation has been shown to generate extreme pH excursions in the pore water, and as a result, the process strongly amplifies the cycling of various pH-sensitive minerals, such sulfide minerals and carbonates. Here we show that e-SOx also strongly influence the early diagenesis of trace metals in coastal sediments. For this, field observations at a shallow subtidal site in the North Sea were combined with dedicated laboratory incubations of repacked sediments. High resolution microsensor profiling (pH, H2S and O2) confirmed the typical geochemical signature of e-SOx both in situ as in the laboratory experiments. Pore water analysis revealed a strong mobilization of both arsenic and cobalt within the electro-active sediment zone. The dissolution of iron sulfides, resulting from the acidification of the pore water by e-SOx, appears to be the main driver for the observed release of cobalt and arsenic. The current generated by e-SOx affects charged species. In this manuscript, we have proposed a new estimation method for the associated current density, based on the rate of sulfide oxidation.Overall, electrogenic sulfur oxidation has a major impact on the cycling of arsenic and cobalt in coastal sediments, and may substantially increase the effluxes of these trace metals to the coastal ocean.

Some intertidal and shallow water polychaetes of the Caribbean coast of Costa Rica.

The polychaete fauna of the Caribbean coast of Costa Rica has been inadequately characterized with only nine species previously reported. Collections of polychaetes from intertidal coralline rocks and several shallow sub-tidal sites on the Caribbean coast of Costa Rica have been examined and 68 species were identified. Of these, 66 are new records for Costa Rica.

Effect of background substrate on recruitment of benthic marine invertebrates to subtidal cobble-filled collectors

The availability of suitable substrate for settlement can alter the behaviour of larvae of benthic marine invertebrates and consequently their settlement patterns. We tested the effect of the background substrate of a site (rocky vs sediment) on recruitment of invertebrates settling into standardized cobble-filled collectors deployed at shallow subtidal sites in the south-western Bay of Fundy, Canada, in 2009 and 2010. The assemblage of invertebrates that settled into the collectors differed significantly in both abundance and species composition between “paired sites” (≈0.003–0.009 km2 in area and 345–861 m apart) with rocky vs sediment background substrate, and markedly less so between paired sites comprised of rocky bottom. Species that are known to be rock-dwelling recruited in greater abundance to cobble-filled collectors at rocky sites, while those known to be sediment-dwelling recruited more to cobble-filled collectors at sediment sites. At the smaller spatial scale of patches of sediment within one site (14–107 m apart), differences in abundance but not species composition were detected between collectors on the two types of background substrate. This study supports the idea that larvae of some species of marine invertebrates respond to substrate at a larger spatial scale than their immediate location of settlement.

The annual gametogenic cycle of the sea anemone Metridium senile from the Gulf of Maine

A year-long study was conducted to quantify the reproductive cycle of the clonal sea anemone, Metridium senile, an important member of the benthic community in the Gulf of Maine (GOM). M. senile is an abundant sessile invertebrate that forms identifiable clonal aggregations that are maintained by pedal laceration but individuals can also reproduce sexually. Specimens were collected monthly from a shallow (< 10 m) subtidal site and the reproductive cycle of male and female anemones was described on a seasonal basis using histological preparation of the gonads and light microscopy. Gametogenesis was determined for both sexes and showed that females undergo spawning during the seasonal transition from summer to fall, while males have mature sperm within testicular cysts in the winter, spring, and summer. Environmental data recorded at the study site shows that the spawning period occurs during a period of peak water temperature (August–September), and rapid gametogenesis occurs during periods of high food availability (April). The low percentage of reproductively mature anemones and the dominance of asexual reproduction in this population are believed to be the result of the high flow regime at this site.

Linking allometric macrobenthic processes to hypoxia using the Peters mass balance model

Eutrophication alters coastal aquatic food web structure and function through effects of organic enrichment and hypoxia. Macrobenthic communities provide good ecological indicators of impacts on ecosystem function induced by eutrophication. However, effective resource management requires validation of ecological indicators through mechanistic links to specific stressors of concern. Organic enrichment along with subsequent hypoxia typically engenders depauperate macrobenthic communities consisting of small short-lived surface-dwelling organisms. Metabolic ecology theory offers great promise for understanding the role of body size within the context of the trophic dynamics of aquatic ecosystems. Body size is a fundamental ecological trait because it underpins vital rates, which can be scaled up to the ecosystem level through allometric laws. Thus, body size-dependent mechanisms can potentially be used to link macrobenthic indicators to ecosystem consequences of organic enrichment and hypoxia. The Peters mass balance model (PMBM) [Peters, R.H., 1983. The Ecological Implications of Body Size. Cambridge University Press, New York, 329 pp.] provides a platform for projecting stress-induced changes in the distribution of biomass among discrete size classes of organisms. In the PMBM, changes in the distribution of biomass ensue over time from differences between gains due to ingestion and losses due to egestion, respiration, and mortality. Production of biomass results when ingestion exceeds combined losses. In the present study, the PMBM was modified to envisage an inhibitory effect of hypoxia on ingestion by: (1) using unique size-specific mortality coefficients in order to realize target inverted biomass-size distributions during model initialization; (2) linking ingestion rate to the hyperbolic relationship between oxygen consumption rate (OCR) and ambient dissolved oxygen (DO); (3) hypothesizing how oxygen regulation ability follows an allometric scaling rule relative to the supply of DO. If food is unlimited, the most straightforward hypothetical connection between the oxygen consumption rate and ingestion assumes a direct proportional relationship under oxygen limitation. Although small organisms can attain much higher mass-specific rates of oxygen consumption than large ones, previous limited research on oxygen consumption relative to oxygen supply implies that large organisms regulate oxygen intake better than small individuals as ambient DO declines. Counterintuitively, linked ingestion deficits would be accordingly higher for small organisms than large organisms under hypoxia. In pilot simulations incorporating hypothetical ingestion deficits due to oxygen limitation within a modified PMBM, contrasting scenarios emulated continuous DO and water temperature conditions experienced at two shallow subtidal sites for 32 d in summer. Resulting ingestion deficits reduced total biomass and favored large size classes. Effects were stronger under the more hypoxic scenario: total biomass was lower and the biomass-size distribution was more uneven among the size classes. Differences were even more evident in terms of an integrative macrobenthic process indicator, the normalized biomass-size spectrum (NBSS). A realistic hypoxia mass balance model (HMBM) must incorporate various allometric effects that are not necessarily expressed in the same direction relative to body size. A useful resource management tool for anticipating effects of environmental stress on aquatic ecosystem function could be developed from the quantification of trophic transfer potential within a refined PMBM.

Two new species of Manunema (Plectida: Peresianidae) from the Arabian Gulf, with notes on the phylogeny of the genus

Two new species of Manunema are described from the coast of Kuwait, one from the intertidal in Kuwait Bay and one found at shallow subtidal sites in the Arabian Gulf. Like other species of Manunema, the two species described here are characterised by coarse striation of the cuticle; a tapered, trunk-like cervical region; a large, elongate oesophageal bulb and corresponding swelling of the body; absence of cervical setae; pedunculate somatic setae; paired outstretched ovaries in females, tubular precloacal supplements in males and a pair of simple, curved spicules in males. Manunema kithara sp. nov. differs from other species in the genus by its characteristic, elongate, double oesophageal bulb which resembles a guitar in shape, being broader in the posterior half, and by the shape and form of the two precloacal supplements in the males. Manunema kuwaitiensis sp. nov. is similar to both M. proboscidis and M. annulata in having a weakly double oesophageal bulb, but differs in the morphology and position of the single precloacal supplement in males.

Diurnal heterogeneity in silicic acid fluxes in shallow coastal sites: Causes and implications

In shallow coastal areas the amplitude and range of benthic silicic acid fluxes can have a significant influence on benthic–pelagic coupling and the functioning of the pelagic system. To explore the oscillation in fluxes over the diurnal cycle and in particular the influence of microphytobenthos (MPB), an experiment was carried out in a shallow subtidal site in the Bay of Brest (France). Benthic chambers were employed over a 48 h period to measure the variability in silicic acid and oxygen fluxes; MPB migration was investigated using a diving Pulse Amplitude Modulated (PAM) fluorometer and uptake rhythms of silicic acid by natural MPB populations were measured using the 32Si isotope. It was discovered that silicic acid fluxes fluctuated greatly throughout the diurnal period resulting in an oscillation in the availability of this nutrient for phytoplankton communities. The uptake of silicic acid by the MPB was quantified for the first time and highlighted a 2-fold increase in the demand from night to afternoon periods. The combined silicic acid uptake and the concentration of cells at the sediment–water interface, forming a dense biofilm of MPB, were postulated to be the main processes reducing effluxes at midday. Our work highlighted the many processes which influence silicic acid effluxes in shallow coastal areas and the possible interaction between uptake and dissolution processes. The variations in benthic fluxes over the diurnal period were comparable to observations reported at the seasonal scale. Therefore, up-scaling hourly flux observations to daily and annual estimates should be undertaken with caution. Further we suggest that the main processes influencing flux oscillations over the diurnal period should be considered when planning sampling strategies and extrapolating to larger time scales.

The influences of census technique on estimating indices of macrofaunal population density in the temperate rocky subtidal zone

Several studies have attempted to compare subtidal animal population estimates obtained in a variety of ways using SCUBA diving and have reported a lot of variation between the estimates obtained. This study investigated individually scale-, tidal-, equipment- and observer-induced variation through analysis of animal population density indices obtained using a number of techniques based on SCUBA diver visual survey. The study was based on shallow subtidal sites on the west coast of Scotland including natural rocky slopes and recently deployed artificial reef sites. Groups of species investigated varied in levels of size, abundance, mobility and detectability. Variability in estimated densities between different sizes of sampling unit, state of tide, type of SCUBA equipment used (open circuit versus rebreather) and observation technique was found to be considerable. A decreasing asymptotic relationship with quadrat size was evident in the majority of cases for estimated density, coefficient of variation and number of samples required to attain a given precision. However, the size of the sampling unit employed had to be large enough to encompass the variability in spatial distribution of the target organism. Improved precision for estimates of cryptic species was obtained from relatively small sampling units, which focused the observer’s attention onto a specific area. Underwater photography as a population estimation technique produced data comparable with that from visual surveys but was limited by the size of sampling unit. The use of remotely deployed infra-red video demonstrated that certain animal species and life-stages actively avoid divers and were largely absent from the diverbased surveys. Although larger quadrats generated more precise estimates, the longer sampling times involved reduced their overall efficiency for most groups surveyed; optimum sampling unit sizes are suggested for particular animal groups.

Impacts of the Jessica oil spill on intertidal and shallow subtidal plants and animals

Densities of fishes, invertebrates and plants at rocky intertidal and shallow subtidal sites were censused 1–2 days prior to the Jessica oil spill and compared with information obtained for the same sites one month after the spill, both for sites impacted by oil and unaffected reference sites. While the availability of pre-spill data made this analysis one of the most powerful to date for testing impacts of oil on shoreline environments, no clear changes attributable to oiling could be identified. Discharged oil appeared to cause very little impact in the intertidal and shallow subtidal zones, with such impacts lying within the range of natural spatial and temporal variation at sites investigated. Factors considered to minimize impact in Galápagos included paucity of fully sheltered shores in spill path, moderate wave action, warm temperature, high levels of sunlight, and mixing of bunker oil with diesel.

Current and wave dynamics in the shallow subtidal: implications to the ecology of understory and surface-canopy kelps

Current and wave properties were studied in the semi-protected waters of the San Juan Archipelago, Washington, at 6 shallow subtidal sites chosen to include a wide range of variability in exposure to both tidal currents and waves. Within each site, 4 to 6 plots measuring roughly 50 to 100 m 2 each were established on nearly horizontal rock platforms at mean depths ranging from 6.1 to 11.0 m, with most plots at mean depths of 7.0 to 9.2 m. Plots were established nominally to minimize within-site variability in hydrodynamics, and were locations of several species of kelp (reported in a companion paper). For more than a year replicate measurements of flow and pressure (mean depth and wave signals) were collected at all sites in 2 Hz bursts 25 cm above the substratum, a height relative to understory kelps, providing a unique, detailed spatial and temporal characterization of a shallow subtidal hydrodynamic regime pertinent to these plants. Despite the intended similarity of plots within sites, local-flow microhabitat remained substantial at scales relative to understory plants, and largely ameliorated differences in tidal signals among most sites. Greater than 50 % of the spatial variability in maximum tidal current speed, and 31 to 44 % of the variance in the duration of periods of calm flow (speeds consistently <20 cm s -1 ), occurred within sites at scales of meters, and not at the larger (km) scales that separated sites. In contrast, wave effects at 7 to 11 m depth were predictable spatially. Significant wave impacts were recorded only at 2 sites characterized by a large, open fetch. Wave-dominated flows were recorded by replicate sensors multiple times within both of these sites, and the strongest instantaneous wave speeds approached 150 to 200 cm s -1 . Maximum wave-generated speeds recorded just above the substratum at these 2 sites were ∼2 to 3x maximum tidal currents. These results help to explain patterns noted in concurrent studies of the population dynamics and morphology of several species of kelp.

DEMOGRAPHY OF FRONDS OF SARGASSUM LAPAZEANUM FROM SOUTHERN BAJA CALIFORNIA, MEXICO, DURING EARLY STAGES

In terms of biomass, Sargassum lapazeanum (Phaeophyceae, Fucales) is one of the most important seaweeds in La Paz Bay, on the southeastern coast of the Baja California Peninsula, Mexico. This species can be found year‐round in shallow subtidal sites. Standing biomass is lowest between fall and winter and highest between spring and summer. We are currently studying the annual demography of fronds as a necessary step to understanding the mechanisms of population regulation. Work is being done on a bed of about 90 m in length and three m in width. At this stage of abstract submission, we can report results for winter only. Random samples were collected in February and in March 2000 to estimate stand biomass, frond density, and size (frond length) structure. In addition, we labeled fronds with numbered plastic tags to estimate their rates of growth and of mortality during this period. Rates of recruitment were estimated from a combined analysis of the above. In February, fronds were all shorter than 6.5 cm. Between February and March, mean total frond density increased from 122 to 776 fronds m−2 (n = 30 quadrats). The mean recruitment rate was 667 fronds m−2, the mean growth rate was 0.5 cm day−1 (n = 60 fronds), and the mean mortality rate was 43 fronds m−2. From last year's preliminary observations, we expect to observe peaks of biomass and of reproduction in late spring, followed by negative growth rates and high mortality rates during summer.

Population biology of Dyopedos monacanthus (Crustacea: Amphipoda) on estuarine soft-bottoms: importance of extended parental care and pelagic movements

The suspension-feeding amphipod Dyopedos monacanthus (Metzger, 1875) is a common epibenthic amphipod that lives on self-constructed ''mud whips'' (built from filamentous algae, detritus and sediment particles) in estuaries of the northern North Atlantic Ocean. The population biology of D. monacanthus at a shallow subtidal site in the Damariscotta River Estuary (Maine, USA) was examined between July 1995 and July 1997. The resident population at the study site was dominated by adult females during most months of the year. High percentages of subadults were found in late summer/early fall. Often, between 10 and 20% of the adult females were paired with males, and the percentage of ovigerous females varied between 40 and 100%, in- dicating continuous reproduction. The percentage of parental females varied between 40 and 80% during most months, but dropped to levels below 20% during summer/early fall. The average size of amphipods on their own mud whips was4 mm during the summer/ early fall, after which it increased continuously to >7.0 mm in March or April, and then dropped again. In March and April, the average number of eggs and juveniles female )1 was100 eggs and 55 juveniles, while during the summer/early fall the average number of eggs female )1 was 1.4 mm) on their mothers' whips in winter/early spring but not in the summer/fall. The average number of amphipods at the study site was low in late summer/early fall ( 3000 individuals m )2 in April 1996 (>1600 individuals m )2 in May 1997), after which densities decreased again. The decrease of the D. monacanthus population at the study site coincided with a strong increase of amphipods found pelagic in the water column. This behavioural shift occurred when temperatures increased and benthic predators became more abundant and active on shallow soft-bottoms, suggesting that D. monacanthus at the study site is strongly aAected by predation. The eAects are direct (by predation on amphipods) and indirect (by reducing du- ration of extended parental care and enhancing pelagic movements). Both extended parental care and pelagic movements are important behavioural traits of D. mon- acanthus (and other marine amphipods), and signifi- cantly aAect its population dynamics.

Storm effects on eelgrass ( Zostera marina L.) and blue mussel ( Mytilus edulis L.) beds

The effects of storms on stands of blue mussels Mytilus edulis L. and eelgrass Zostera marina L. at a shallow subtidal site in the Western Baltic were examined. In mixed Zostera/Mytilus stands, the experimental removal of the eelgrass canopy strongly increased the risk of mussel dislodgement during a moderate storm with a median return time of 6.5 months. In contrast, no protection of mussels by eelgrass was found during a series of three intense storms, each of which had a median return time of > 11.5 yr. Also, mussel presence had no effect on the persistence of eelgrass patches (assessed as shoot density) during the moderate storm. The eelgrass meadow had a strong impact on dispersal of mussel clumps under non-storm conditions (wind velocities < 10 m/s). A transplantation experiment with small M. edulis patches showed that drifting mussel aggregates had a tendency to come to rest within eelgrass meadows and may initiate new mussel beds. The results partly explain the preference of blue mussel M. edulis beds for eelgrass Z. marina habitats at shallow subtidal sites of the Western Baltic.

Colonization of meiobenthos in oil-contaminated subtidal sands in the lower Chesapeake Bay

In-situ manipulative experiments were conducted over a 3-month period (May–August 1980) to examine the rate at which meiobenthos colonizes oiled and untreated azoic fine sands at a shallow subtidal site in the lower York River, Virginia. Three concentrations of fresh Prudhoe Bay crude oil were added to sediments: 100, 2 500 and 10000 mg oil kg-1 dry wt sediment. Untreated azoic and natural sediments served as controls. Within 16 d, meiofauna densities in all treatments were comparable to natural populations in surface oxidized sediments, but densities fluctuated greatly during the remainder of the sampling period. Nematodes slowly colonized the subsurface anoxic sediments below the redox potential discontinuity (RPD); some less common species did not significantly recover below the RPD in the two more heavily oiled treatments. Analysis of nematode community composition by reciprocal averaging ordination and numerical classification revealed generally lower abundances, but no distinct differences, in species composition in the oiled substrates as compared to untreated and natural community controls. Ordination of sequential samples suggested that the nematode species assemblages in the untreated controls fully recovered from these small-scale disturbances by 90 d. Life history characteritics and frequent tidal transport combine to make estuarine meiobenthos highly resilient following disturbance. Contrary to prior recolonization studies, a successional sequence was found for the colonizing nematodes which may be analogous to models of macrobenthic colonization (e.g. McCall, 1975). The comesomatid nematode Sabatieria pulchra, which is frequently dominant in polluted sediments, colonized relatively late in the experiment. Consequently, stress resistance and resilience may not be as coincident in meiofauna as in macrofauna because of differences in factors affecting their dispersal.

Occurrence of seeds and seedlings of Thalassia testudinum banks ex König in the Florida keys (U.S.A.)

Established Thalassia seedlings were found in a shallow subtidal site in the Florida Keys ( x ̄ = 1 seedling 12 m −2 ). The 1979 seed crop was probably large since one beach strand site of 0.5 km long and 5 m wide contained on average, 14.8 seedlings m −2. Seeds in these quantities could be used for habitat restoration purposes. The seed crop is probably highly variable on an annual basis.