Reintroduction of eelgrass ( Zostera marina ) in the Dutch Wadden Sea; Review of research and suggestions for management measures

Eelgras (Zostera marina L.) in the Dutch Wadden Sea historically covered an area varying from 65–150 km² in the eulittoral as well as the sublittoral zones. At present, this area comprises less than 1 km² eulittoral eelgrass stands, with an associated decrease in habitat diversity. The causes for this decline are presumably connected with the ‘wasting disease’ and the closure of the former Zuiderzee in the early 1930s resulting in increased tidal range and increased currents. After a slight recovery of the eelgrass populations on the intertidal flats a definite decline started in the early 1970s, possibly connected to increased turbidity. The present water quality and turbidity do not negatively influence eelgrass growth up to a depth of at least 0.6m below Mean Sea Level. Based on mesocosm experiments and field experiments it is concluded that re-establishment of eelgrass should be possible in sheltered bays and on unexposed tidal falts. The most suitable depths for a reintroduction are those between 0 and 20–40 cm below mean sea level.     

Interactions between two introduced species: Zostera japonica (dwarf eelgrass) facilitates itself and reduces condition of Ruditapes philippinarum (Manila clam) on intertidal flats

Dwarf eelgrass (duckgrass; Zostera japonica) and Manila clams (Ruditapes philippinarum) are two introduced species that co-occur on intertidal flats of the northeast Pacific. Through factorial manipulation of clam (0, 62.5, 125 clams m⁻²) and eelgrass density (present, removed by hand, harrowed), we examined intra- and interspecific effects on performance, as well as modification of the physical environment. The presence of eelgrass reduced water flow by up to 40% and was also observed to retain water at low tide, which may ameliorate desiccation and explain why eelgrass grew faster in the presence of conspecifics (positive feedback). Although shell growth of small (20–50 mm) clams was not consistently affected by either treatment in this 2-month experiment, clam condition improved when eelgrass was removed. Reciprocally, clams at aquaculture densities had no effect on eelgrass growth, clam growth and condition, or porewater nutrients. Overall, only Z. japonica demonstrated strong population-level interactions. Interspecific results support an emerging paradigm that invasive marine ecosystem engineers often negatively affect infauna. Positive feedbacks for Z. japonica may characterize its intraspecific effects particularly at the stressful intertidal elevation of this study (+1 m above mean lower low water).     

The contribution of seagrass beds (<Emphasis Type="Italic">Zostera noltii</Emphasis>) to the function of tidal flats as a juvenile habitat for dominant,mobile epibenthos in the Wadden Sea

Since the substantial loss of subtidal eelgrass (Zostera marina L.) in the 1930s, seagrass beds in the Wadden Sea are limited to the intertidal zone and dominated by Z. noltii Hornem. This study deals with the effect of vegetated tidal flats on quantities of mobile epifauna and proves empirically the function of seagrass canopies as a refuge for marine animals remaining in the intertidal zone at ebb tide. Drop-trap samples were taken in the Sylt-Rømø Bight, a shallow tidal basin in the northern Wadden Sea, on vegetated and unvegetated tidal flats during July and August 2002, and during the entire growth period of Z. noltii from May to September in 2003. The species composition in Z. noltii and bare sand flats showed minor differences since only two isopod species (Idotea baltica and I. chelipes) occurred on Z. noltii flats exclusively. Juvenile shore crabs (Carcinus maenas L.), brown shrimps (Crangon crangon L.) and common gobies (Pomatoschistus microps Krøyer) were also found abundantly on bare sand flats. However, the results showed significantly higher abundances and production of these dominant species on vegetated tidal flats. Additionally, the analyses of faunal size classes indicated higher percentages of small individuals in the seagrass bed during the entire sampling period. Despite drastic diurnal fluctuations of dissolved oxygen at low tide, faunal density in the residual water layer remaining in seagrass canopies at ebb tide was found to be consistently higher than that found in artificially created tide-pool units. Although species composition of mobile epifauna did not basically differ between vegetated and unvegetated tidal flats, Z. noltii beds are considered to contribute quantitatively to the function of tidal flats, as an extended juvenile habitat for some of the most important species of the Wadden Sea food web.Communicated by O. Kinne, Oldendorf/Luhe     

Abundance of small cetaceans in waters of the central Spanish Mediterranean

Seasonal aerial surveys were conducted in the waters of the central Spanish Mediterranean from 2001 to 2003 using the line transect sampling methodology to estimate cetacean abundance. The density of the three most abundant species, striped dolphin (Stenella coeruleoalba), bottlenose dolphin (Tursiops truncatus) and Risso’s dolphins (Grampus griseus), was estimated. In the case of the first two species, the density was estimated accounting for the proportion of submerged animals, while for Risso’s dolphin only the surface density could be estimated. The striped dolphin was the most abundant species in the study area with a mean density of 0.489 dolphins km⁻² (95% CI = 0.339–0.705) and a mean abundance of 15,778 dolphins (95% CI = 10,940–22,756). This density is comparable to that obtained in the International Ligurian Sea Cetacean Sanctuary. Striped dolphins were observed throughout the whole year and no seasonal changes in the density were detected. The mean density of bottlenose dolphins was an order of magnitude lower than that of striped dolphins (0.041 dolphins km⁻²; 95% CI = 0.023–0.075) with a mean abundance of 1,333 dolphins (95% CI = 739–2,407). The Risso’s dolphin had a surface estimated density of 0.015 dolphins km⁻² (95% CI = 0.005–0.046) and a mean abundance of 493 dolphins (95% CI = 162–1,498). These results provide valuable biological information useful to develop conservation plans and establish a baseline for future population trend studies.     

Distribution and abundance of finless porpoise in the Inland Sea of Japan

Aerial sighting surveys were conducted in 2000 to evaluate the distribution and abundance of finless porpoise Neophocaena phocaenoides in the Inland Sea of Japan. We flew 60 north–south transects (2,218 km in total) at intervals of 6.43 km over the study area between 131°3′ and 134°59′ E. In total, 148 groups were detected by two observers. The average group size was 1.56 individuals. The effective strip half-width of each observer was estimated to be 107 m (coefficient of variation [CV] = 8.26%). Porpoise abundance was estimated at 7,572 individuals and the density was as low as 0.506 individuals/km² (CV = 17.3%). The sea is among the Japanese waters with the lowest density in spite of its favorable topographical conditions. In the western stratum of the study area, where the estimated density was 1.31 individuals/km², we observed a relatively regular distribution. In the central-eastern stratum, which had an estimated density of 0.208 individuals/km², we confirmed a clumped distribution that was restricted to inshore waters or near islands. No individuals were observed in waters between 132°51′ and 133°11′E, and between 133°43′ and 133°59′E, where sand dredging and other human activities have been active, suggesting that habitat fragmentation has occurred.     

Are Wadden Sea tidal systems with a higher tidal range more resilient against sea level rise?

Accelerated sea level rise may have serious implications for the Wadden Sea ecosystem in its present state. If sediment accumulation rates on the extensive intertidal flats stay behind sea level rise, the flats will eventually submerge. Drowning of the flats has negative consequences for nature conservation and for coastal risk management. Based upon an evaluation of steady state relations for Wadden Sea tidal basins, Hofstede (Zeitschrift für Geomorphologie 59(3): 377-391, 2015) postulated that the capacity of these basins to balance sea level rise by accumulation on intertidal flats seems positively related to mean tidal range. In the present study, morphodynamical simulations with a numerical model were performed for two tidal basins in the German Wadden Sea to verify the empirically established hypothesis. The following conclusions are established. Larger mean tidal range improves the capacity of Wadden Sea tidal basins to balance sea level rise. Wadden Sea intertidal flats are effective sediment sinks and seem quite resilient against (higher rates of) sea level rise. Finally, subtidal gullies may constitute a significant sediment source for accumulation on intertidal flats in response to sea level rise. With respect to the limited comparability of the two investigated tidal systems, morphodynamical modelling of all Wadden Sea tidal systems should be conducted.     

Effect of periphyton grazing by Hydrobia ulvae on the growth of Zostera noltii on a tidal flat in the Dutch Wadden Sea

The decrease of the intertidal seagrass Zostera noltii in the Dutch Wadden Sea may have been the result of enhanced periphyton load due to eutrophication. Decrease of this seagrass species coincided with an increase in the mudsnail Hydrobia ulvae. Feeding of this mudsnail on periphyton may have partly counteracted an increase of periphyton biomass. Exclosure experiments on seagrass stands in the Dutch Wadden Sea in 1987 showed that density of periphyton on leaves of Z. noltii decreased significantly with increasing density of grazers. An increased density of mudsnails significantly enhanced the density and biomass of seagrass, in particular of the below ground parts. Since this seagrass species survives winter in temperate climate zones mainly by means of rhizomes, grazing may also influence the seagrass dynamics in the long term. Results of the experiment were in agreement with field observations on coinciding low densities of mudsnails and high densities of fouling of seagrass stands on the tidal flats of western Wadden Sea in the late 1970s.     

Abundance and variability of microorganisms and transparent exopolymer particles across the ice–water interface of melting first-year sea ice in the Laptev Sea (Arctic)

The distribution and abundance of transparent exopolymer particles (TEP) was determined in and below pack ice of the Laptev Sea from July to September 1995. Samples were collected from the lowermost 10 cm of ice floes and at 10 cm below the ice–water interface. Abundance of bacteria, protists and TEP was determined, and the sea ice–water boundary layer was characterized using temperature, salinity and molecular viscous shear stress. TEP, with a distinct size distribution signal, were found in highest concentrations inside the sea ice, ranging from not detectable to 16 cm² l⁻¹ (median: 2.9 cm² l⁻¹). In the water, concentrations were one order of magnitude lower, ranged from below detection to 2.7 cm² l⁻¹ (median: 0.2 cm² l⁻¹) and decreased after the middle of August, whereas abundances of autotrophic flagellates (AF), diatoms, heterotrophic flagellates (HF) and ciliates increased. The abundance of TEP decreased with its size in all samples following a power law relationship. The relation of TEP to the microbial community differed between the sea ice and water, being positively correlated with bacteria and diatoms in the ice and negatively correlated with HF in the sea water. The presence of a pycnocline significantly influenced the abundance of organisms, diatom composition and TEP concentrations. Pennate diatoms dominated by Nitzschia frigida were most abundant inside the ice. Though bacteria have the potential to produce exopolymeric substances (EPS), the results of this study indicate that the majority of TEP at the ice–water interface in first-year Arctic summer pack ice are produced by diatoms. Received: 19 August 1999 / Accepted: 4 July 2000     

Experiments on resuspension of natural microphytobenthos populations

Laboratory experiments on resuspension of benthic diatoms were performed in 1988 on intact cores of sediment obtained from two transects in the Ems estuary (Dutch Wadden Sea, NW Europe). Diatoms were most abundant in sediment from sheltered stations. In the experiments, which were performed in a small carousel system, the percentage of resuspended diatoms increased concomitantly with angular velocity of the agitating paddles. Maximum microphytobenthos resuspension in terms of cell numbers was up to 45% of the total population present in the 0.5-cm top layer, and up to 11% in terms of chlorophylla. Maximum sediment resuspension was 6%. The results show that the presence of a diatom film (numerous cells on the sediment surface) increases sediment stability, thereby suppressing resuspension of sediment and diatoms. However, if the sediment contains many fine particles and much detritus, sediment stability decreases, leading to increased resuspension.These results support the contention that low detritus content and large numbers stabilize the surface layer of tidal flats, whereas high detritus content and fewer cells (<2 × 10⁵ cm^–2) do not. The stabilization is thought to be dependent on a combination of differences in sediment consolidation and in bottom roughness caused by the presence or absence of diatom films.     

Recent distribution and size structure of gelatinous organisms in the southern Black Sea and their interactions with fish catches

The spatial distributions of gelatinous organism were studied during three cruises in 2006–2007. These were the first such studies conducted in the southern Black Sea for last decade. Additionally, the different methods of estimating gelatinous organism biomass presently in use were compared and recent situation of pelagic fishery interacting with zooplankton in the Black Sea were overviewed. Biomasses and abundances of both invading Mnemiopsis leidyi, and the resident Aurelia aurita and Pleurobrachia pileus were remarkably reduced (total gelatinous biomass: 417–537 g m⁻² in May, June, and 150 g m⁻² in October) as compared with those before the 1997 with invasion Beroe ovata in the southern Black Sea. Specimens of B. ovata were detected only in October when individuals with lengths of 10–50 mm comprised 93% of its population. Length–weight based biomasses were significantly variable for all species depending on the size structure in time. Larger sized individuals (>30 mm) of M. leidyi appeared as compared with those before year 1997. The ecosystem of the Black Sea has been suppressed both bottom–up through decreased eutrophication and top–down through reduced grazing on mesozooplankton by M. leidyi that are in turn controlled by grazing by B. ovata. The catch of anchovy was abruptly reduced to 120,000 tons due presumably to the considerably increased catch of its predator, the bonito (64,000 tons) in the year 2005.     

Spatial and temporal pattern in seagrass community composition and productivity in south Florida

We document the distribution and abundance of seagrasses, as well as the intra-annual temporal patterns in the abundance of seagrasses and the productivity of the nearshore dominant seagrass (Thalassia testudinum) in the south Florida region. At least one species of seagrass was present at 80.8% of 874 randomly chosen mapping sites, delimiting 12,800 km² of seagrass beds in the 17,000-km² survey area. Halophila decipiens had the greatest range in the study area; it was found to occur over 7,500 km². The range of T. testudinum was almost as extensive (6,400 km²), followed by Syringodium filiforme (4,400 km²), Halodule wrightii (3,000 km²) and Halophila engelmanni (50 km²). The seasonal maxima of standing crop was about 32% higher than the yearly mean. The productivity of T. testudinum was both temporally and spatially variable. Yearly mean areal productivity averaged 0.70 g m⁻²day⁻¹, with a range of 0.05–3.29 g m⁻² day⁻¹. Specific productivity ranged between 3.2 and 34.2 mg g⁻¹ day⁻¹, with a mean of 18.3 mg g⁻¹ day⁻¹. Annual peaks in specific productivity occurred in August, and minima in February. Integrating the standing crop for the study area gives an estimate of 1.4 × 1011 g T. testudinum and 3.6 × 10¹⁰ g S. filiforme, which translate to a yearly production of 9.4 × 10¹¹ g T. testudinum leaves and 2.4 × 10¹¹ g S. filiforme leaves. We assessed the efficacy of rapid visual surveys for estimating abundance of seagrasses in south Florida by comparing these results to measures of leaf biomass for T. testudinum and S. filiforme. Our rapid visual surveys proved useful for quantifying seagrass abundance, and the data presented in this paper serve as a benchmark against which future change in the system can be quantified. Received: 30 January 2000 / Accepted: 24 July 2000     

Life history and production of the mysid <Emphasis Type="Italic">Orientomysis robusta</Emphasis>: high <Emphasis Type="Italic">P/B</Emphasis> ratio in a shallow warm-temperate habitat of the Sea of Japan

Although mysids play important roles in marine food chains, studies on their production are scarce, especially for warm-water species. We investigated life history and production of Orientomysis robusta in a shallow warm-temperate habitat of the Sea of Japan. Its spawning and recruitment occurred throughout the year; 19 overlapping cohorts were recognizable over an annual cycle. The summer cohorts recruited in July–September exhibited rapid growth, early maturity, small brood size, and small body size. A converse set of life history traits characterized the autumn–winter cohorts recruited in October–March. The spring cohorts recruited in April–June had intermediate characteristics of both cohorts. Life spans were 19–33, 21–48, and 69–138 days for summer, spring, and autumn–winter cohorts, respectively, and mortality rates were high for spring and summer cohorts, especially during June–August but were low for autumn–winter cohorts. Production calculated from the summation of growth increments was 488.8 mg DW m⁻² year⁻¹ with an annual P/B ratio of 21.26. The short life span seems to be responsible for such an extremely high P/B ratio. A method not requiring recognition and tracking cohorts gave similar values (534.0 mg DW m⁻² year⁻¹ and 20.49). The close agreement in production values between the two methods indicates our estimates are valid.     

Significance of salinity and silicon levels for growth of a formerly estuarine eelgrass (Zostera marina) population (Lake Grevelingen, The Netherlands)

Since the early 1980s, the eelgrass, Zostera marina L., population in the saline Lake Gevelingen, The Netherlands, is rapidly declining. An earlier study, in which long-term data on eelgrass coverage in this former estuary were correlated with several environmental variables, showed only one significant correlation: coverage was positively related to water column silicon levels. In addition, a negative correlation with salinity was observed, but this was not significant. In the present study, the effect of silicon and the effect of salinity on the development of Z. marina were investigated experimentally. Enhancement of dissolved silicon concentrations in the water did not stimulate Z. marina above-ground production or an increase in final above- and below-ground biomass. The highly significant correlation between eelgrass coverage and water column silicon levels, thus, remains to be explained. The results of the growth experiments did, however, demonstrate a clear effect of salinity on Z. marina growth. Plants cultured at 22 psu showed a higher production of shoots and leaves, resulting in more above-ground biomass, than plants grown at 32 psu. In addition, below-ground biomass was also higher at 22 psu. Measurements of chlorophyll a fluorescence, performed with a PAM-fluorometer, indicated a reduction of photosynthesis in the high-salinity treatments. Thus, low salinity stimulates development of Z. marina from Lake Grevelingen. Eelgrass from such a historically estuarine area may be more sensitive to high salinities than other, more marine populations. Recovery of the autochthonous eelgrass population is expected to be favoured when the estuarine conditions of the seagrass area are re-established, or when restoration programmes are carried out with allochthonous ecotypes that are less sensitive to high salinities. Received: 23 June 1998 / Accepted: 19 November 1998     

Distribution and abundance of moon jellyfish (Aurelia aurita) and its zooplankton food in the Black Sea

The distribution of moon jellyfish (Aurelia aurita Linnaeus, 1758) in the Black Sea was determined from plankton samples collected above the anoxic zone (maximum depth 200 m) in the summer, winter and spring during 1991–1995. Distribution was patchy. Average biomass ranged from 98 to 380 g m⁻², and abundance varied from 2 to 14 individuals m⁻². Biomass and abundance peaked in late spring and summer. The distribution was correlated with hydrographic features in the Black Sea, with higher concentrations occurring at the peripheries of anticyclonic eddies. Centers of the two main cyclonic gyres generally had a low biomass of A. aurita. From July 1992 to March 1995, the populations were largely concentrated in offshore regions. A. aurita were confined to the upper part of the mixed layer. Smaller A. aurita (≤1 cm) were present in early spring (March), and individuals reached maximum size in the summer. Release of the epyhrae occurred in spring on the northwestern shelf of the sea when the seawater temperature was 11–12 °C. Microscopic analysis of stomach contents showed that copepods and mollusks form their main diet. Received: 3 September 2000 / Accepted: 29 September 2000     

Light regime in an Arctic fjord: a study related to stratospheric ozone depletion as a basis for determination of UV effects on algal growth

Solar radiation as a primary abiotic factor affecting productivity of seaweeds was monitored in the Arctic Kongsfjord on Spitsbergen from 1996 to 1998. The radiation was measured in air and underwater, with special emphasis on the UV-B (ultraviolet B, 280–320 nm) radiation, which may increase under conditions of stratospheric ozone depletion. The recorded irradiances were related to ozone concentrations measured concurrently in the atmosphere above the Kongsfjord with a balloon-carried ozone probe and by TOMS satellite. For comparison, an ozone index (a spectroradiometrically determined irradiance of a wavelength dependent on ozone concentration, standardized to a non-affected wavelength) was used to indicate the total ozone concentration present in the atmosphere. Weather conditions and, hence, solar irradiance measured at ground level were seldom stable throughout the study. UV-B irradiation was clearly dependent on the actual ozone concentration in the atmosphere with a maximal fluence rate of downward irradiance of 0.27 W m⁻² on the ground and a maximal daily fluence (radiation exposure) of 23.3 kJ m⁻². To characterize the water body, the light transmittance, temperature and salinity were monitored at two different locations: (1) at a sheltered shallow-water bay and (2) at a wave-exposed, deep-water location within the Kongsfjord. During the clearest water conditions in spring, the vertical attenuation coefficient (K _d) for photosynthetically active radiation (PAR) was 0.12 m⁻¹ and for UV-B 0.34 m⁻¹. In spring, coinciding with low temperatures and clear water conditions, the harmful UV radiation penetrated deeply into the water column and the threshold irradiance negatively affecting primary plant productivity was still found at about 5–6 m depth. The water body in spring was characterized as a Jerlov coastal water type 1. With increasing temperature in summer, snow layers and glacier ice melted, resulting in a high discharge of turbid fresh water into the fjord. This caused a stratification in the optical features, the salinity and temperature of the water body. During melt-water input, a turbid freshwater layer was formed above the more dense sea water. Under these conditions, light attenuation was stronger than defined for a Jerlov coastal water type 9. Solar radiation was strongly attenuated in the first few metres of the water column. Consequently, organisms in deeper water are protected against harmful UV-B radiation. In the surface water, turbidity decreased when rising tide caused an advection of clearer oceanic water. In the course of the summer season, salinity continuously decreased and water temperature increased particularly in shallow water regions. The impact of global climate change on the radiation conditions under water and its effects on primary production of seaweeds are discussed, since organisms in the eulittoral and upper sublittoral zones are affected by UV radiation throughout the polar day. In clearer water conditions during spring, this may also apply to organisms inhabiting greater depths. Received: 20 June 2000 / Accepted: 17 October 2000     

Early reproduction and development of dominant calanoid copepods in the sea ice zone of the Barents Sea—need for a change of paradigms?

Reproduction and growth of the dominant copepods Calanus finmarchicus, C. glacialis, C. hyperboreus and Pseudocalanus minutus were studied on transects across the sea ice zone of the northern Barents Sea in May and June 1997. C. glacialis and C. finmarchicus were numerically dominant and also the largest component of the biomass. C. hyperboreus was rather rare. Moderate levels of phytoplankton and eventually high concentrations of ice algae supported maximum egg production rates of 53.6 and 48.5 eggs female^–1 day^–1 of C. glacialis in May and June, respectively. Results of incubation experiments were supported by a tremendous abundance of C. glacialis eggs in the water column ranging from 7×10³ to 4.4×10⁴ m^–2 in May and from 9.8×10³ to a maximum of 9.7×10⁴ m^–2 in June. In contrast, C. finmarchicus spawned only in the vicinity of the ice edge, at a maximum rate of 30 eggs female^–1 day^–1. Egg sacs of P. minutus were often observed in the preserved samples, but contained only few eggs, which may be due to loss during sampling. The presence of considerable concentrations of young stages in May and June indicated successful recruitment of C. glacialis and P. minutus. Back calculation using published stage duration estimates indicates March/April as the begin of the reproductive and growth period for these species under the first-year ice of the Barents Sea. Hence, secondary production in the study area starts at the same time as in open water regions and polynyas in the northern North Atlantic. Although the role of ice algae in the nutrition of copepods was not clarified here, the significant relationship between phytoplankton chlorophyll and egg production of C. glacialis suggests that high reproductive activity has already been achieved at moderate food concentrations.Communicated by O. Kinne, Oldendorf/Luhe     

Preliminary patterns of distribution and abundance of loggerhead sea turtles,<Emphasis Type="Italic"> Caretta caretta</Emphasis>, around Columbretes Islands Marine Reserve,Spanish Mediterranean

Aerial surveys were conducted to estimate the abundance and distribution of loggerhead turtles (Caretta caretta) in the Columbretes Islands Marine Reserve and surrounding waters (western Mediterranean). Four surveys were carried out during 2000 and 2001, following the line transect methodology. Loggerheads appeared to be present at high densities in the area throughout the whole year, although density varied between seasons, being more abundant during the spring. Mean density in the study area was 0.322 turtles/km² (range 0.200–0.516) and the mean abundance was 1,324 turtles (range 825–2,124). The turtles were distributed homogeneously throughout the study area, we found no difference in loggerhead density between the water around the reserve and that in the rest of the study area. Current conservation measures planned by the local authorities, which include increasing the area of the reserve, would be very positive for the conservation of this stock.Communicated by S.A. Poulet, Roscoff     

Drastic decline of an extensive eelgrass bed in Nova Scotia due to the activity of the invasive green crab (Carcinus maenas)

In 2001–2002, there were severe declines of eelgrass (Zostera marina) in estuaries along the Nova Scotia coast of the southern Gulf of St. Lawrence. We examined the relationship between the recent invasion of European green crabs (Carcinus maenas) and the eelgrass decline in Benoit Cove in Tracadie Harbour. The abnormally abundant eelgrass wrack consisted mainly of entire shoots, not the usual blades. Three separate methods yielded similar estimates of the rate of shoot removal from the eelgrass beds (direct quadrat counts, “mark–recapture” of tagged shoots in the eelgrass bed, and rate of shoot appearance in the shore wrack). From 14 July to 8 September 2002, the shoot density in the bed fell by about 75 %. Similar rates of decline occurred in crab enclosures (4.4 crabs m^?2) placed in the eelgrass bed. Green crab foraging, involving the tearing of shoots and the digging of large pits, was the reason for the drastic decline of the eelgrass bed in Benoit Cove. It is clear that an invasion of green crabs to a region where they had not previously existed can both destroy eelgrass beds and restrict their recovery.     

UV- and salinity-induced oxidative effects in the marine diatom <Emphasis Type="Italic">Cylindrotheca closterium</Emphasis> during simulated emersion

The diatom Cylindrotheca closterium was exposed to transient light- and osmotic conditions as occur during its tidal emersion. The objective was to analyze how this simulated emersion contributes to the production of active oxygen species (AOS) and via this, to oxidative cell damage. Light- and salinity conditions were varied in factorial combination: low light (no UVB) or high light (unweighted UVB-dose rates of respectively 0.01; 0.07; 0.24; 1.03 W m⁻²) at normal (30 psu) or high salinity (60 psu). UVB (0.01–0.24 W m⁻²) and high salinity had a significant, negative effect on the photosynthetic efficiencies ΔF/F _m’ (steady-state quantum yield) and F _v/F _m (maximum yield). UVB at 1.03 W m⁻² (15 kJ m⁻² d⁻¹) almost arrested electron transport. At ecologically relevant UVB levels, i.e. below 0.24 W m⁻² (≈3.4 kJ m⁻² d⁻¹) with UVB:PAR<0.4:100 (PAR photosynthetically active radiation) only dynamic photoinhibition was observed (protection via heat dissipation). Non-photochemical quenching was positively correlated with the de-epoxidation of diadinoxanthin (DD) to diatoxanthin (DT). A decreasing ratio DT/(DD+DT) after 4 h of UVB at >0.07 W m⁻² and at 60 psu indicated a reversal of the diatom xanthophyll cycle (diminished photoprotection) which may be caused by an enhanced AOS production. Oxidative stress and -damage to C. closterium cells were assessed applying fluorescent indicator dyes, via confocal microscopy and quantitative image analysis. AOS production rates (cellular DCF fluorescence) were stimulated by UV, and were ~50% higher at 60 psu. AOS production decreased with an increasing pre-exposure (0–4 h) to normal UVB (0.24 W m⁻²), which indicated a stimulation of the antioxidative defence. Non-protein thiols (indicator CMF) and glutathione pools (HPLC-analyzed) decreased with UVB-dose rates (0.01–0.24 W m⁻²), most likely due to AOS-mediated thiol oxidation. Hypersalinity (60 psu) and UVB (0.01–0.24 W m⁻²) caused membrane depolarization (dye DIBAC₄(3)) and phospholipid hydrolysis (phospholipase A₂ dye: bis-BODIPY FL-C₁₁-PC). AOS production may have diminished the membrane polarity, and peroxidized the membrane lipids (HPLC-analyzed malondialdehyde) which enhanced PLA₂ activity. The dyes indicated an increased oxidative (lipid) damage at a 15% inhibition of photosynthesis in this diatom, at UVB levels and salinities that can be expected in situ during its periodic tidal emersion.     

Aggregations and offshore movements as indicators of spawning activity of bonefish (Albula vulpes) in The Bahamas

To identify the timing and location of spawning activity for bonefish (Albula spp.) in the Bahamian archipelago, we used an acoustic telemetry array spanning 44?km² of shallow tidal creeks, flats, and adjacent deeper coastal waters near Cape Eleuthera. In two successive years, we surgically implanted transmitters in male and female bonefish (n?=?60) and examined their movement patterns within the array. Eight bonefish surgically implanted with transmitters as part of an earlier study were also tracked. In 2009, the telemetry information was complemented with snorkeling observations, underwater video, and manual tracking of the same acoustically tagged fish, as well as fish (n?=?3) gastrically implanted with continuous transmitters. During a period of 4?C7?days spanning the full and new moons, primarily between October and May, bonefish moved from their typical shallow flats and aggregated at sites in close proximity to the deep water drop-off of the Exuma Sound. Localized movements of the large schools of bonefish (often?>1,000 fish) at these presumptive pre-spawning aggregation sites included brief trips (<8?h) just after sunset until just prior to sunrise to the abyssal wall at the edge of the Exuma Sound (i.e.,?>1,000?m depth). Tagged bonefish detected at these aggregation sites were subsequently detected back in the tidal creeks and coastal flats shortly after new and full moons and remained at these more typical shallow sites (i.e.,?<2?m depth). Although we did not directly observe spawning events, we did observe ventral nudging and porpoising behaviors, which are potentially associated with courtship. Timing of the observed movements and possible courtship behaviors was coincident with periods when gametes were well developed. Collectively, our study provides the first objective evidence suggesting that the aggregation and seasonal migration of bonefish to deep shelf environments during certain moon phases is for spawning.