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

Eelgrass (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 flats. The most suitable depths for a reintroduction are those between 0 and 20–40 cm below mean sea level.     

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.     

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     

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.     

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     

The effect of temperature on the development and hatching of resting eggs of non-indigenous predatory cladoceran Cercopagis pengoi in the Gulf of Finland, Baltic Sea

The effect of temperature on the development and hatching of resting eggs of the Ponto-Caspian Cercopagis pengoi was studied experimentally in the Gulf of Finland, Baltic Sea. Morphological changes were monitored as the development of the resting eggs proceeded. Sexual reproduction of the C. pengoi population in the Gulf of Finland was evaluated by combining the data from hatching experiments and resting egg abundances in the sediment. Development time of resting eggs was dependent on temperature: increase in the temperature shortened the time needed until hatching. Hatching success was also dependent on incubation temperature. Almost sixfold increase in hatching success was detected when temperature increased two degrees above the storage temperature. Average resting egg abundances varied between 0.16 and 0.49 eggs cm⁻³ in the 0–6 cm sediment layer.     

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     

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).     

Sediment ammonium availability and eelgrass (Zostera marina) growth

The interaction of sediment ammonium (NH ₄ ⁺ ) availability and eelgrass (Zostera marina L.) growth, biomass and photosynthesis was investigated using controlled environment and in-situ manipulations of pore water ammonium concentrations. Sediment diffusers were used to create pore water diffusion gradients to fertilize and deplete ammonium levels in sediments with intact eelgrass rhizospheres. Between October, 1982 and September, 1983 controlled environment experiments using plants from shallow (1.3 m) and deep (5.5 m) stations in a Great Harbor, Woods Hole, Massachusetts, USA eelgrass meadow along with in-situ experiments at these stations provided a range of sediment ammonium concentrations between 0.1 and 10 mM (adsorbed+interstitial NH ₄ ⁺ ). The results of the in-situ experiments indicate that nitrogen limitation of eelgrass growth does not occur in the Great Harbor eelgrass meadow. A comparison of NH ₄ ⁺ regeneration rates and eelgrass nitrogen requirements indicates an excess of nitrogen supply over demand and provides an explanation for the lack of response to the manipulations. Results of controlled environment experiments combined with in-situ results suggest that sediment ammonium pool concentrations above approximately 100 mol NH ₄ ⁺ per liter of sediment (interstitial only) saturate the growth response of Zostera marina.     

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.     

Life cycle of the copepod Calanus hyperboreus in the Greenland Sea

The seasonal ontogenetic migration of the Arctic copepod Calanus hyperboreus was described from surface-to-bottom hauls in the central Greenland Sea Gyre (GSG) and in the Westspitsbergen Current (WSC). All stages except females spent the winter below 500 m in the GSG and below 1000 m in the WSC. Seasonal ascent begins in April, and descent in July. For the C.␣hyperboreus population an active downward transport of 8.1 g m⁻² dry weight during 8 months of overwintering was estimated, similar to flux rates of particulate matter in sediment traps. Seasonal distribution of biomass was determined from weight measurements of single stages. Annual means varied from 4.0 to 9.2 g m⁻² in two different years in the GSG and were 1.1 in 1 year in the WSC. The life cycle in the Greenland Sea was reconstructed from field data on stage composition, vertical distribution, reproduction, and moult cycle phase from tooth development of CV. Laboratory experiments were conducted on the moulting of CIV and CV in fall. A 3-year (males) and 3- to 4-year (females) life cycle is proposed for the GSG and 2 to 3 years for the WSC. However, the small number of young larvae and the incomplete spring ascent by older copepodites observed in the WSC cast doubt on the reproductive success in the WSC. A suite of physiological strategies and adaptations performed by the developmental stages support survival of this species in harsh environments. Received: 25 January 1997 / Accepted: 11 February 1997     

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.     

Tiger shark (Galeocerdo cuvier) abundance and growth in a subtropical embayment: evidence from 7 years of standardized fishing effort

The tiger shark (Galeocerdo cuvier Peron and Lesueur 1822) is a widely distributed predator with a broad diet and the potential to affect marine community structure, yet information on local patterns of abundance for this species is lacking. Tiger shark catch data were gathered over 7 years of tag and release research fishing (1991–2000, 2002–2004) in Shark Bay, Western Australia (25°45′S, 113°44′E). Sharks were caught using drumlines deployed in six permanent zones (~3 km² in area). Fishing effort was standardized across days and months, and catch rates on hooks were expressed as the number of sharks caught h⁻¹. A total of 449 individual tiger sharks was captured; 29 were recaptured. Tiger shark catch rate showed seasonal periodicity, being higher during the warm season (Sep–May) than during the cold season (Jun–Aug), and was marked by inter-annual variability. The most striking feature of the catch data was a consistent pattern of slow, continuous variation within each year from a peak during the height of the warm season (February) to a trough in the cold season (July). Annual growth rates of recaptured individuals were generally consistent with estimates from other regions, but exceeded those for populations elsewhere for sharks >275 cm fork length (FL), perhaps because mature sharks in the study area rely heavily on large prey. The data suggest that (1) the threat of predation faced by animals consumed by tiger sharks fluctuates dramatically within and between years, and (2) efforts to monitor large shark abundance should be extensive enough to detect inter-annual variation and sufficiently intensive to account for intra-annual trends.     

Functional responses of copepod nauplii using a high efficiency gut fluorescence technique

To investigate copepod nauplii ingestion rates on phytoplankton, we have adapted the traditional gut fluorescence technique as it can be used with lower gut pigment concentrations. With the improved technique, laboratory experiments were performed to estimate functional responses for nauplii of Calanus helgolandicus and Centropages typicus. Nauplii were raised from eggs to copepodites and the experiments were performed with stages NIV-NV. Gut evacuation rates and ingestion rates were measured on Isochrysis galbana at different concentrations. Specific ingestion rates ranged between 0.038–0.244 μg C μg⁻¹ nauplii C d⁻¹ for C. typicus and 0.041–1.412 μg C μg⁻¹ nauplii C d⁻¹ for C. helgolandicus. Both species showed a type III functional response, reaching a saturation concentration at around 600 μgC l⁻¹ for C. typicus and 800 μgC l⁻¹ for C. helgolandicus. An erratum to this article can be found at     

Population biology and life strategies of Chlorophthalmus agassizii Bonaparte, 1840 (Pisces: Osteichthyes) in the Mediterranean Sea

The population biology and life strategies of Chlorophthalmus agassizii were studied in the Ionian Sea (eastern–central Mediterranean) using the data collected during the experimental trawl surveys carried out from 1995 to 2000. Depth-related trends of both density and size were found. With depth the former decreased while the latter increased. A typical bigger–deeper phenomenon was detected: young-of-the-year individuals occur on the shelf during autumn–winter months and move towards bathyal bottoms as they grow. The sampled population was made up of several size–age groups. The maximum age of 8 years was identified by means of otolith readings. The Von Bertalanffy growth parameters were estimated from the age–length key (L _∞=189.04±5.401 mm; k=0.24±0.021; t _o=−1.20±0.132; φ′=3.94) and modal progression analysis (L _∞=218.33±18.397 mm; k=0.164±0.028; t _o=−1.694±0.171; φ′=3.89). Reproduction of this monoecious fish was observed during summer–early autumn. Considering the female portion of the gonad, the size at attainment of 50% maturity was 115 mm TL. The corresponding age is within the third year of life. The simultaneous occurrence of oocytes in different development stages was shown in the ovary. Both the asynchronous ovary and oocyte size distribution indicate that C. agassizii spawns more than once during a reproductive season (batch spawner). Functional fecundity (on average 3,018 hydrated oocytes) was between 37 and 69% of the absolute fecundity and increased significantly with the individual size. Since adult specimens are mostly distributed on the slope, eggs and larvae develop in epipelagic waters and migrate in-shore where juvenile forms recruit on the shelf. Juveniles migrate ontogenetically towards deeper bottoms and after 2–3 years start to reproduce annually within a life span greater than 10 years.     

Thermal effects on swimming activity and habitat choice in juvenile Pacific cod (Gadus macrocephalus)

The behavioral responses of fishes to temperature variation have received less attention than physiological responses, despite their direct implications for predator–prey dynamics in aquatic ecosystems. In this paper, we describe the temperature dependence of swimming performance and behavioral characteristics of juvenile Pacific cod (Gadus macrocephalus; 75–125 mm total length). Maximum swimming speeds increased with temperature and body size. Routine swimming speeds of Pacific cod in small groups of similarly sized fish (N = 6) increased with body size and were 34 % faster at 9 °C than at 2 °C. The response to temperature was opposite that previously described for juvenile walleye pollock (Theragra chalcogramma), reflecting species-specific differences in behavioral responses. In a separate experiment, we demonstrated the effect of temperature on habitat selection of juvenile Pacific cod: Use of an artificial eelgrass patch in a 5-m-long laboratory tank was significantly greater at 9 °C than at 2 °C. These results illustrate that temperature affects a range of behavioral traits that play important roles in determining the frequency and outcomes of predator–prey interactions.     

Meiofauna response to a dynamic river plume front

 The benthic response to a plume front was studied in two areas of the northern Adriatic (Mediterranean Sea) differently influenced by the Po River freshwater input. Sediment samples were collected in June 1996 and February 1997 from 12 stations. The adopted sampling strategy was able to identify the front line in real time by satellite images and to locate sampling stations along an inner–outer plume gradient in order to cover the benthic area beneath the river plume, where enhanced biological production was expected, and open-sea sediments not directly influenced by freshwater inputs. Meiofaunal parameters were compared to the physical conditions and to phytodetritus inputs, organic matter accumulation and bacterial secondary production. The sediments of the Adriatic Sea were characterised by high concentrations of phytopigments (0.6 to 13.9 μg g⁻¹ for chlorophyll a and 1.2 to 17.7 μg g⁻¹ for phaeopigments) and biopolymeric organic carbon (0.15 to 3.02 mg g⁻¹). The plume system extended for a large sector of the northern Adriatic. In the northern area, a large and highly dynamic plume area was coupled with a sediment organic matter concentration significantly higher than in open-sea sediments. In the southern sector, where the plume area and the front line did not change markedly during the year, plume–benthic coupling was evident only in the sediments beneath the front, and corresponded to phaeopigment accumulation. Bacterial parameters and secondary production were high and significantly higher in the frontal area than at open-sea stations. Meiofauna density (1342 to 8541 ind. 10 cm⁻²) did not change either by season or between areas and was significantly correlated with phaeopigments and bacterial secondary production. Meiofauna displayed different responses to plume inputs in the two sampling areas. In the northern sector, meiofauna density was coupled with organic matter distribution and displayed highest values beneath the plume. In the southern sector, the densities of copepods, turbellarians and kinorhynchs displayed highest values under the front in summer, and the same applied to total meiofauna density in winter. Juvenile decapods and copepod nauplii significantly increased their densities in sediments beneath the front. Data presented in the present study suggest that plume inputs and frontal systems, enhancing phytodetritus accumulation and benthic bacterial response, might influence density, composition and distribution of meiofaunal assemblages. As river plumes are highly variable systems affecting the trophic characteristics of the sediments underneath, their dynamics should be considered when analysing mesoscale spatial changes of meiofaunal assemblages. Received: 30 November 1999 / Accepted: 24 May 2000     

Abundance,distribution and prey composition of scyphomedusae in the southern North Sea

The annual cycle of abundance and distribution of the scyphozoan medusae Aurelia aurita, Cyanea lamarckii, C. capillata and Chrysaora hysoscella were studied in the southern North Sea in 2004 and 2005. Three different patterns of seasonal occurrence of medusae were distinguished: (1) the early occurring C. lamarckii (February–August), (2) C. capillata and A. aurita (April–August) and (3) the late appearing C. hysoscella (July/August–September). Cyanea lamarckii was the most frequently encountered species in this study; its highest mean abundance was 1.8 ± 2.7 ind. 100 m⁻³. The prey spectra of C. lamarckii, C. capillata and C. hysoscella contained several copepod and other crustacean species and thus make them potential competitors with fish larvae. Medusae in this study also consumed fish eggs and larvae, including clupeids, in all months analysed. Although peak spawning of sprat (Sprattus sprattus) coincides with the maximum abundance of medusae (May–June) the relative low abundance of all medusae species in this study makes jellyfish predation unlikely to be a factor controlling sprat recruitment in the time frame investigated.     

Annual cycle of biomass of a threatened population of the intertidal seagrass Zostera japonica in Hong Kong

The phenology and primary productivity of a population of Zostera japonica (Aschers. & Graebn.) threatened by the construction of Hong Kong's new international airport were studied over a 12-month period. The need to conserve the population, and the small leaf size of Z. japonica rendered traditional destructive or marking techniques inapplicable for percentage cover and biomass estimation. A nondestructive method based on image analysis techniques was therefore devised for repeated estimation of percentage cover, biomass and leaf area index. This technique, which involved random quadrat sampling, photographic recording and image analysis, was able to provide data on the three parameters with acceptable precision and was cost-effective in the field. Z. japonica demonstrated a strongly seasonal cycle of vegetative growth, with different patterns for leaf density (peak in March) and overall bed area (peak in June). Total (above- and below-ground) net primary productivity was estimated at between 344 and 688 g AFDW m⁻² yr⁻¹. Percentage cover of Z. japonica was negatively correlated with total suspended solids (TSS) in the water column while total bed area was negatively correlated with water salinity. Increased sedimentation associated with the new airport project was identified as one important factor affecting the growth of the seagrass, as TSS reached the high level of ≈1 g DW l⁻¹ during the first half of the study period. Sediment traps set in the beds also recorded potential sedimentation rates at between 2.89 and 14.5 mg cm⁻² d⁻¹. This high turbidity resulted in a sharp decrease in the density of Clithon spp., the dominant grazers of epiphytic algae on Z. japonica. Effects of sedimentation and shading on growth of Z. japonica were investigated by field manipulative experiments. Experimental increase of sedimentation rate and shade both resulted in larger decreases in percentage cover and above-ground AFDW compared with the control. Received: 3 March 1997 / Accepted: 14 March 1997     

The effects of temperature and salinity on reproductive success of <Emphasis Type="Italic">Temora longicornis</Emphasis> in the Baltic Sea: a copepod coping with a tough situation

At specific locations within the Baltic Sea, thermoclines and haloclines can create rapid spatial and temporal changes in temperature (T) and salinity (S) exceeding 10°C and 9 psu with seasonal ranges in temperature exceeding 20°C. These wide ranges in abiotic factors affect the distribution and abundance of Baltic Sea copepods via species-specific, physiological-based impacts on vital rates. In this laboratory study, we characterized the influence of T and S on aspects of reproductive success and naupliar survival of a southwestern Baltic population of Temora longicornis (Copepoda: Calanoida). First, using ad libitum feeding conditions, we measured egg production (EP, no. of eggs female⁻¹ day⁻¹) at 12 different temperatures between 2.5 and 24°C, observing the highest mean EP at 16.9°C (12 eggs female⁻¹ day⁻¹). Next, the effect of S on EP and hatching success (HS, %) was quantified at 12°C for cohorts that had been acclimated to either 8, 14, 20 or 26 psu and tested at each of five salinities (8, 14, 20, 26 and 32 psu). The mean EP was highest for (and maximum EP similar among) 14, 20 and 26 psu cohorts when tested at their acclimation salinity whereas EP was lower at other salinities. For adults reared at 8 psu, a commonly encountered salinity in Baltic surface waters, EP was relatively low at all test salinities—a pattern indicative of osmotic stress. When incubated at 12°C and 15 different salinities between 0 and 34 psu, HS increased asymptotically with increasing S and was maximal (82.6–84.3%) between 24 and 26 psu. However, HS did depend upon the adult acclimation salinity. Finally, the 48-h survival of nauplii hatched and reared at 14 psu at one of six different temperatures (10, 12, 14, 16, 18 and 20°C) was measured after exposure to a novel salinity (either 7 or 20 psu). Upon exposure to 7 psu, 48-h naupliar mortality increased with increasing temperature, ranging from 26.7% at 10°C to 63.2% at 20°C. In contrast, after exposure to 20 psu, mortality was relatively low at all temperatures (1.7% at 10°C and ≤26.7% for all other temperatures). An intra-specific comparison of EP for three different T. longicornis populations revealed markedly different temperature optima and clearly demonstrated the negative impact of brackish (Baltic) salinities. Our results provide estimates of reproductive success and early survival of T. longicornis to the wide ranges of temperatures and salinities that will aid ongoing biophysical modeling examining climate impacts on this species within the Baltic Sea.