Journal of Sea Research

Published by Elsevier
Print ISSN: 1385-1101
In the present study we tested the hypothesis that environmental concentrations of the antifouling agent Irgarol 1051, as measured in coastal Western European waters, affect marine phytoplankton performance. The impact of Irgarol was investigated in the phytoplankton species Thalassiosira weissflogii, Emiliania huxleyi, Tetraselmis sp. and Fibrocapsa japonica. EC50 concentrations for growth, effective quantum yield of PSII and viability were calculated from dose response relationships established during 72 h exposures to six Irgarol concentrations. Furthermore, the biological recuperation from a temporary exposure to a high Irgarol concentration (39.47 nM l− 1) was monitored. Growth rates and effective quantum yield were strongly affected by Irgarol, however viability loss was never observed. EC50 values differed five fold between species and ranged from 0.43 to 2.38 nM for effective quantum yield and from 0.46 to 2.44 nM for growth rate. For all species, complete biological recuperation was shown within 3–4 days after the Irgarol treatment, both for effective quantum yield and growth rate. All calculated EC50 values and EC20 values fall within the Irgarol concentration range measured in Western European coastal waters. We therefore conclude that present day Irgarol 1051 levels may affect the in situ performance of marine phytoplankton in this area.
Differences in fish community structure between different estuaries, lagoons and bays can be very large, and generalisations are complicated by the use of a wide variety of sampling methods. In the present study, fish communities of subtidal seagrass beds and sandy seabeds in 13 marine embayments of a single Caribbean island were therefore sampled using a uniform method. The objective of the study was to determine whether the seagrass and sandy seabed habitats of various embayments are characterised by typical fish assemblages which differ in terms of taxa (species, families), size classes (life stages) and functional groups (ecological species groups, feeding time and diet). This was linked to the hypothesis that differences in fish assemblages between habitats in different embayments are larger at taxonomic levels than at the level of functional groups. A second objective was to determine the most useful discriminating features between the two habitat types. The above hypothesis was rejected, since differences in fish assemblages from different seagrass and sandy seabed sites did not increase from functional to taxonomic level, but from size class to diet/species to family/feeding time to ecological species group. However, the seagrass and sandy seabed habitats could each be characterised by typical fish assemblages which differed in taxonomical and functional group composition, irrespective of differences in environmental and biotic variables between the embayments in which these habitats were situated. The two habitat types could be best characterised on the basis of fish family, ecological species group, feeding time and size distribution. Seagrass beds mainly harboured nocturnally active nursery species (Haemulidae, Lutjanidae, etc.), whose relative abundance was related to vegetation (mainly seagrass) cover. Sandy seabeds mainly harboured diurnally active bay species (Gerreidae, etc.) whose relative abundance was related to cover of bare sand. Similarities in taxonomical and functional traits of fish species predicted whether they occurred more abundantly in seagrass beds or in sandy seabeds.
Stock collapses have occurred worldwide. The most frequently cited cause is over-fishing, suggesting that fisheries management has been ineffective in controlling exploitation rates. The progression of a fishery from an over-exploited to a collapsed state involves impairment of the reproductive capacity of the target species, i.e. recruitment over-fishing. In many cases, this occurs by reduction of the spawning stock biomass (SSB) through the systematic elimination of spawning components within a stock complex. While operational definitions of minimum levels of SSB have been developed, they have seldom been applied and never adopted in a Canadian groundfish management context. The answer to the question of how much is enough to perpetuate a stock under exploitation has been illusive. Serebryakov [J. Cons. Int. Explor. Mer, 47 (1990) 267] has advocated definition of critical levels of SSB based on survival rates (R/SSB). We review his method and discuss the utility of the approach. An alternative approach to the problem of estimating minimum SSB is through a fundamental revision of the traditional stock and recruitment relationship. Explicit theoretical SSB thresholds below which reproduction/recruitment is severely impaired based upon density-dependent mating success (or Allee effects) is considered a superior approach to the question of how much is enough because of its ecological grounding. However, the successful application of this approach will require re-definition of the space/time scales of the management unit. Finally, support is growing for the establishment of closed areas or “no-take zones” as an alternative approach to managing the problems of fishing a stock complex by enabling sub-populations to escape fishing. While the expected benefits of areas protected from fishing are numerous, clear demonstrations of benefits of such areas in marine temperate ecosystems are lacking. In fact, unintended negative consequences may result from such actions.
Two haptophycean strains were isolated from field samples collected in 2001 in Belgian coastal waters (southern North Sea) during the Phaeocystis monitoring program of the AMORE Project. The morphology and pigment composition of these two strains, one identified as Phaeocystis globosa and the other as Imantonia rotunda, were carefully examined. The comparative analysis of their pigment signature revealed the presence of two fucoxanthin derivatives, 19′-butanoyloxyfucoxanthin and 19′-hexanoyloxyfucoxanthin (but-fuco and hex-fuco) in I. rotunda, which were undetectable in P. globosa. A further comparison of pigments and phytoplankton from field samples showed no significant correlation between hex-fuco concentration and P. globosa biomass in the water column. Low concentrations of this pigment were, however, detectable before and at the end of the Phaeocystis bloom. The presence of I. rotunda in the area, overlooked by light microscopy, but isolated in pure culture from field samples, might explain the presence of this pigment. We conclude that hex-fuco is not the appropriate pigment to estimate Phaeocystis abundance and trace its trophic fate in Belgian coastal waters. These results also indicate that pigment analysis should be coupled with a precise identification of phytoplankton taxa present in field samples.
The Calanus finmarchicus population of the North Sea has collapsed since the late 1950s, while abundance of temperate Atlantic and neritic species groups has risen. These changes are explored in relation to the changing environment of the North Sea. Non-parametric regression methods are used throughout the study, in order to compare the spatial, long-term and seasonal dynamics of the changes in both biotic (e.g. C. finmarchicus) and physical variables (e.g. temperature, salinity, and stratification). The fall in the population of C. finmarchicus has coincided with a long-term freshening and warming of the eastern North Sea and a long-term increase in the salinity of the western North Sea. At the same time the prevalence of temperate Atlantic and neritic zooplankton species has risen. The changes may be explained by differing origins of Atlantic water entering the North Sea since the late 1950s.
Narragansett Bay, Rhode Island, USA showing location (41° 34′ 07″ N, 71° 23′ 31″ W) of long-term monitoring site in the lower West Passage of the bay.
Comparison of monthly mean Skeletonema spp. abundance before and after an August 1980 change point. Abundance values are monthly means for the periods prior to and after the change point year. p-value is for Wilcoxon 2-sample test comparing mean abundance before and after change point. Ratio of mean abundance after:before change point year given Skeletonema spp. abundance (cells ml − 1 )
Significant (p b 0.05) associations (Pearson correlation coefficients) between the long- term Skeletonema trend and annual or quarterly means of environmental variables. 1Q, 2Q, 3Q, 4Q refer to first through fourth quarter mean values; integrated chlorophyll and nutrient values calculated from surface, mid-depth and bottom observations; n refers to number of years or quarters in each comparison
Mean values and 95% confidence intervals of environmental variables having significantly different (p b 0.10) means during winter-spring, summer and autumn bloom years. Mean values in winter-spring, summer, or autumn bloom pattern years were compared by Kruskal-Wallis test. 1Q, 2Q, 3Q, 4Q refer to first through fourth quarter mean values; integrated nutrient values calculated from surface, mid-depth (ca. 4 m) and bottom (ca. 8 m) observations. p-value and Kruskal-Wallis H statistic and significantly different pairings as indicated by Dunn's multiple comparison post tests are shown for each variable; post test notation: 1=winter-spring, 2=summer. 3=autumn bloom pattern
A 38-year time series (January 1959 to May 1997) of weekly observations of abundance of the marine diatom Skeletonema spp. and related plankton habitat parameters in lower Narragansett Bay, Rhode Island was compiled and analyzed. A statistical change point test identified two different abundance regimes characterized by a ca. 45% decline in Skeletonema abundance. In the first 260 months of the time series (January 1959 to August 1980), the mean deseasonalized Skeletonema abundance was 2137 cells ml− 1, which declined to 1128 cells ml− 1 in the final 201 months (August 1980 to May 1997) of the time series. The decline was greatest during the winter–spring bloom period; Skeletonema abundance in March declined from a mean of ca. 3300 cells ml− 1 prior to the change-point to ca. 700 cells ml− 1 after the change point.
Research vessel surveys showed that yellowtail flounder (Pleuronectes ferruginea) on the Grand Bank, off New-foundland in the Northwest Atlantic, declined in abundance between the 1970s and the mid 1990s. The northern limit of distribution decreased substantially in the late 1980s and early 1990s, coincident with large declines in population abundance and a decrease in bottom water temperatures. This range contraction continued into the mid 1990s, despite a stabilization of the population size and a reversal of the cooling trend. In 1995, this species was rarely found on the northern Grand Bank. The area west of the Southeast Shoal on the southern Grand Bank had relatively high densities of yellowtail flounder throughout the period studied. The area occupied by the stock was positively correlated with stock abundance from surveys, but not with bottom temperatures from these same surveys. We conclude that the contraction in the area of distribution for this stock to the preferred habitat around the Southeast Shoal is primarily a function of low stock size, which resulted from increased fishing activity in the mid to late 1980s.
Dissolved and particulate trace metals (Cu, Cd, Pb, Zn, Ni, Fe and Mn) measured at six stations along the Scheldt estuary in October/November 1978 are compared with more recent data. Based on Ca content in the suspended matter, three distinct geochemical regions could be distinguished: the upper estuary (salinity 1–7) dominated by fluvial mud, mid-estuary (salinity 7–17) where the composition of the suspended matter remained relatively constant, and the lower estuary where marine mud prevailed. Re-suspension of sediments is the major factor controlling the composition of the particles in the upstream region. Anoxic conditions prevailed in the upper part of the estuary extending to a salinity of 15 in 1978, while at present the seaward boundary of the anoxic water body is located at less saline waters. Furthermore, the present-day metal load is much lower than in 1978. As a consequence of the changed situation, maxima in dissolved concentrations of redox-sensitive metals in the mid/lower estuary have moved as well, which affects the trace metal re-distribution pattern. In the anoxic zone, exchange processes between dissolved and particulate metal fractions were strongly redox regulated, with Fe and Mn as excellent examples. Iron was removed from the dissolved phase in the early stages of mixing resulting in an increase in the suspended particulate matter of the leachable `non-residual' Fe fraction from 2 to 3.5%. Due to its slower kinetics, removal of Mn from solution occurred in mid-estuary where oxygen concentrations increased. Cu, Cd and Zn on the contrary were mobilised from the suspended particles during estuarine mixing. External inputs of Pb, and to a lesser extent of Cu, in the lower estuary resulted in the increase of their particulate and the dissolved concentrations. Calculated Kd (distribution coefficient) values were used to assess the redistribution between the dissolved and particulate phase of the investigated metals. Due to the existence of the anoxic water body in the upper estuary, the importance of redox processes in determining the Kd values could be demonstrated. The sequence of Kd values in the upper estuary (Fe, Cd, Zn, Pb > Cu > Ni, Mn) is significantly different from that in the lower estuary (Fe > Mn > Pb, Ni, Zn, Cu, Cd). Thus, in such a dynamic estuary single metal-specific Kd values cannot be used to describe redistribution processes.
The spatial and temporal distribution of trace metals (Cd, Pb, Zn and Cu), polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) was studied to assess trends in the sediment of the Dutch coastal zone (<63-μm fraction) from 1981 to 1996. In general, the distributions showed a distinct zonation, with highest concentrations close to the coast near freshwater outlets and where dredged material had been dumped. Over time, the zone with high concentrations narrowed and moved towards the coast. The median of the percentage of the <63-μm fraction in the surface sediments did not change significantly in the Dutch coastal zone between 1981 and 1996. In that period, the median concentrations of Cd, Cu, Zn and Pb fell by 71, 38, 49 and 53%, respectively, in the area close to the coast (<20 km) north of the outlet of the Rhine. South of this outlet the median Cd, Cu and Zn concentrations fell by 45, 29 and 17%, respectively, and in the open sea area (>20 km offshore) the median concentrations for Cu and Zn fell by 35 and 18%, respectively. The largest reduction (80%) of the median concentration of PCBs occurred in the surface sediment in the open sea area (>20 km offshore) between 1986 and 1996. During this period the median PCBs concentration fell by nearly 70% close to the coast. The median PAH concentration fell by only 26% north of the outlet of the Rhine (<20 km from the coast) between 1986 and 1996. In the other areas there was no significant change. Possible reasons are that the main sources of PAHs are in the North Sea itself and that the load of PAHs from dredged material dumped in the Dutch coastal zone increased during the period studied. The reduction of the concentrations of metals and organic micropollutants in the surface sediments of the Dutch coastal zone is explained by a conservative physical mixing model, in which the previously polluted silt fraction in the surface sediments is replaced by less polluted suspended matter. There is a time lag of several years between the trend in the load of Cd and PCBs and the observed field concentration. The half life of the silt fraction and associated substances in the surface sediments is 1.9. Taking the uncertainties in the loads into account, the calculated surface sediment concentrations for Cd and PCBs agree fairly well with the field measurements. It is concluded that there are three main reasons for the decline in the metal, PCB and PAH concentrations in the highly dynamic surface sediments of the Dutch coastal zone: (1) the decrease in the load from various sources; (2) the sedimentation and mixing of less polluted suspended matter into the active sediment layer; and (3) the washing out of previously deposited more heavily polluted particles.
Samples of the demersal fish fauna have been collected by beam trawl from the coastal waters of northwest Europe (49–57°N, 8°W–9°E) by the UK, Netherlands, Germany and Belgium, since 1990, during the third quarter of the year. Changes in community structure within small spatial scales were subtle as species compositions formed part of a continuum over the entire continental shelf. Populations of low diversity were particularly evident in the German Bight and on the North Sea continental coast, where dab Limanda limanda were abundant. In the Channel and to the west of the UK the demersal assemblages were more species-rich than in the North Sea and, although dab was still an important member of the underlying fish assemblage, the abundance of other species, especially poor cod Trisopterus minutus, solenette Buglossidium luteum, plaice Pleuronectes platessa, and the lesser weever, Echiichthys vipera, allowed a range of different groups to be identified. Despite the greater species diversity in this westerly region only eight out of a total nineteen flatfish species were found in abundance. The dominance of different species in different size classes was a key feature of the community structure. Flatfish were the largest group by weight in the smaller-length classes (<30 cm), and in western areas the elasmobranchs dominated the larger-size classes. Observed patterns in community structure were partly explained by the zoogeography of the region and the presence of the British Isles at the boundary between two faunal types. The additional influence on demersal populations of depth and substrate type, which may regulate the abundance of flatfish at key stages in their life history, was also discussed. In addition to these natural processes, recent increases in fishing effort are thought to have affected the structure of the demersal assemblage, and an examination of aggregated length-frequency distributions from these surveys tends to support this conclusion. Without further information on the distribution of fishing effort, it is not possible to separate the influence of natural faunal changes between regions from that of artificial changes caused by fishing activity.
The development of the microbial community was studied in the northern German Wadden Sea and adjacent parts of the North Sea in three synoptic surveys between 9 February and 8 March 1996. During the study period the abundance of bacteria remained relatively constant, with mean values ranging from 0.7 to 0.8×105 cells cm−3. The mean bacterial activity, measured as thymidine uptake, slightly increased from 11 to 16×10−12 mol thymidine h−1 dm−3, but high rates of up to 70×10−12 mol h−1 dm−3 were also observed within inner tidal basins and near freshwater in-flows. These sites were categorised as having a rapid winter turn-over of organic material. Temperature adaptation was tested in incubations at 2, 12 and 22°C. At the beginning of the study period, only the organisms of the near-shore stations showed highest activity at the lowest temperature, but in the course of the experiment the number of cold-adapted organisms increased steadily. This shift from cold tolerance to cold preference was probably facilitated by the supply of organic material from various sources and indicates the development of the microbial community simultaneously with the onset of the growing season.
Turbot (Psetta maxima Linnaeus) is a high value commercially exploited marine flatfish which occurs in European waters, from the Northeast Atlantic to the Arctic Circle, the Baltic and Mediterranean Sea. In Ireland, turbot are the most valuable commercial non-quota species. Very little is known about their population dynamics in the wild, in particular during the sandy beach nursery phase of the life history. In 2000, a survey was established to assess flatfish species on nursery grounds on the west coast of Ireland. Eleven sandy beaches were assessed for 0+ turbot by beach seining, over an eight year period (2000–2007) during the months of August and September. The objective of the study was to estimate juvenile turbot abundance and size structure to determine if any spatial and annual trends existed. Large scale variability in the recruitment of fish to nursery grounds may be indicative of fluctuations in the adult stock. Turbot were found to recruit to five beaches consistently over the eight year period. Temporal and spatial variability in the relative abundance and length of turbot was discerned, with no apparent overall trend. However, certain nursery grounds were shown in most of the years examined to support higher abundances of turbot in comparison to other areas over the eight year period. Turbot abundances on nursery grounds were significantly correlated with mean spring sea temperatures during the pelagic stage. The condition of turbot did not significantly differ on an annual or spatial scale. Mean densities of 0+ turbot along the Irish coast were found to be similar and at times higher than other areas in Europe, ranging from 0.1 (± 0.3) individuals 1000 m− 2 to 18.5 (± 6.9) individuals 1000 m− 2. Mean turbot total length on beaches ranged from 3.8 cm (± 0.6) to 6.6 cm (± 4.3). The observed spatial and temporal variability in abundance and length highlights the need for long-term studies when assessing juvenile flatfish populations. Results from the present study have provided much needed baseline data on wild juvenile turbot populations which is severely lacking for this species both on an Irish and on a European scale.
Macrobenthic soft bottom community assemblages were studied from December 2006 to December 2007 at Paracas Bay (Atenas Beach, 13°45' S, 76°17' W, Peru), including the period of the tsunami of 15 August 2007, providing a unique opportunity to assess the effects of this type of natural disturbance in soft bottom marine ecosystems. The results show that the tsunami affected the soft bottom community assemblage by changing the sediment granulometry, the biomass and abundance of epifaunal species, and by increasing the silt proportion. The event affected all functional groups, resulting in an increased numbers and biomass of filter feeder and grazers that were followed soon after by predators during the post-tsunami period. A similar pattern was observed for biomass and the abundance of infaunal species without changing the functional groups. The slow recovery observed after four months was likely related to the loss of sediment stability. In summary, there were differences in the structure and composition of soft bottom macrofaunal assemblages before and after the earthquake and posterior tsunami disturbance, forcing benthic communities to initiate re-colonisation processes.
Regular phytoplankton research in the Marsdiep started in the early 1970s as a curiosity-driven project fitting in the International Biological Programme (IBP). Interest was primarily in seasonal variation in species composition, biomass and primary production. Initially our monitoring was not intended to extend beyond the IBP programme, which ended in 1971. However, the example and support of Jan Beukema and the interesting links with his work on secondary production in the Wadden Sea were decisive in its continuation. As a result, we now have a time series of well over 30 years. We had the good luck to start in a period of relatively low phytoplankton values, whereas in the late 1970s biomass and primary production doubled in a short period. Duration of the Phaeocystis blooms, which regularly occur in spring and early summer in Dutch coastal waters, also increased. Light limitation plays a dominant role in turbid coastal waters. Our Secchi disc data up to 1985 did not indicate changes in turbidity, and therefore increases were seen as a eutrophication phenomenon. What would then be more natural than to expect phytoplankton to decrease again with the lowering of phosphate values in the 1980s and 1990s as a result of the successful cleaning of the Rhine? The data up to 1992 did not show any effect on phytoplankton of this de-eutrophication. Annual primary production during the period 1992–2000 decreased from a peak value of >400 gC m−2 in 1994 to ca. 250 now. Over the entire period 1974–2000, a slight increase in turbidity was observed. Phosphate concentrations have dropped a little further since 1992, but as could be expected, dissolved nitrogen (nitrate, nitrite and ammonia) and reactive silica did not show changes since the mid 1970s; inputs from freshwater and the atmosphere have not decreased. In accordance with the decrease of primary production, also chlorophyll-a and Phaeocystis cell numbers have dropped since 1994. These first signs of a decrease in phytoplankton parameters probably related to de-eutrophication make continuation of our Marsdiep monitoring highly interesting. No lengthening of the growing season of phytoplankton was observed comparable to that observed in terrestrial vegetation and related to global warming, but Phaeocystis growth seems to start earlier now. Our phytoplankton research covers only a small part of the period of man-induced changes of the Wadden Sea ecosystem, which started already in the Middle Ages. Certainly the recent overfishing of filter feeders such as cockles and mussels has affected phytoplankton, but also the cultivation of mussels and introduction of exotics such as Crassostrea gigas and Ensis directus must have influenced phytoplankton. The Marsdiep phytoplankton time series has proved to be useful in the continued ecosystem research in the western Wadden Sea. Continuation of this time series will also be helpful to study natural versus human-induced variations in this area.
The distribution of megabenthic epifauna (invertebrates) in the Balearic Basin (western Mediterranean) has been analyzed at depths between 427 and 2265 m after compiling samplings performed in 1985–1992 and 2007–2008 with an OTSB-14 bottom trawl. 84 epibenthic taxa of invertebrates (excluded decapod crustaceans) were collected. Epibenthic assemblages were organized in five groups (n-MDS analyses) as a function of increasing depth: upper slope assemblage, U, hauls between 427 and 660 m; middle slope assemblages M1 and M2, hauls between 663–876 m and 864–1412 m, respectively; lower slope assemblages L1 and L2, hauls between 1488–1789 m and 1798–2265 m, respectively). We found significant differences in assemblage composition between all depth-adjacent pairs of groups. Trends in the distribution of biomass vs. depth and within assemblages varied when hauls taken over insular were compared to those over mainland slopes. Over insular slopes we found (n-MDS) only four distinct depth assemblages, with significant differences between all depth-adjacent group pairs, except between L1 and L2. Over the mainland slope, two peaks of biomass situated at U (427–660 m) and at L1 (1488–1789 m) were clearly identified, attributable to the echinoid Brissopsis lyrifera and holothurian Molpadia musculus at U and to the synallactid holothurian Mesothuria intestinalis at L1. The distribution of biomass vs. depth on insular slopes did not follow this pattern, showing no significant biomass peak below 1000 m and a total biomass an order of magnitude lower than adjacent to the mainland. After compiling available environmental data over the mainland slope off Barcelona, we found coincidence between the peak biomass of Mesothuria intestinalis and: i) a significant increase of labile OM (%OrgC, C/N, hydrolizable aminoacids–EHAA, and the EHAA/THAA-total hydrolizable aminoacids-ratio) over 1600 m; and ii) an increase of turbidity and T at 1500–1600 m in February 2008. We suggest that such OM inputs must likely be associated to the formation of nepheloid layers close to submarine canyons, probably associated with oceanographic processes in deep water masses in the area. This would explain why aggregations of M. intestinalis were linked to the mainland part of the Balearic basin, with highest densities located south of canyons. If hotspots of biomass as cited here for M. intestinalis are regulated by factors such as river inputs, both natural climatic changes (e.g. changes in rainfall regimes) and human impact (e.g. river damming) may affect deep-Mediterranean communities below 1000 m.
The Wadden Sea (North Sea, Europe) is a shallow coastal sea with high benthic and pelagic primary production rates. To date, no studies have been carried out in the Wadden Sea that were specifically designed to study the relation between pelagic respiration and production by comparable methods. Because previous studies have suggested that the import of primary-produced pelagic organic matter is important for benthic Wadden Sea carbon budgets, we hypothesised that on an annual average the northern Wadden Sea water column is autotrophic. To test this hypothesis, we studied annual dynamics of primary production and respiration at a pelagic station in a shallow tidal basin (List Tidal Basin, northern Wadden Sea). Since water depth strongly influences production estimates, we calculated primary production rates per unit area in two ways: on the basis of the mean water depth (2.7 m) and on the basis of 1 m depth intervals and their respective spatial extent in the List Tidal Basin. The latter more precise estimate yielded an annual primary production of 146 g C m− 2 y− 1. Estimates based on the mean water depth resulted in a ∼40% higher annual rate of 204 g C m− 2 y− 1. The total annual pelagic respiration was 50 g C m− 2 y− 1. The P/R ratio varied between seasons: from February to October the water column was autotrophic, with the highest P/R ratio of ∼4–5 during the diatom spring bloom in April/May. In autumn and winter the water column was heterotrophic. On an annual average, the water column of the List Tidal Basin was autotrophic (P/R ∼3). We suggest that a large fraction of the pelagic produced organic matter was respired locally in the sediment.
The Benthos 373 plankton silhouette camera was one of the first instruments used to study the in situ particle size spectra of flocculated material. Based on a concept originally developed by H. Edgerton for the study of live zooplankton samples, the relatively non-invasive nature of the camera has made it well suited for the study of flocculated suspensions in the coastal zone. Results from studies using the camera showed clearly the dynamic nature of particle flocculation in both fresh and salt water environments. Originally limited by the time required to carry out image analysis of the photographs obtained, recent advances in the digitization of photo negatives and image analysis software have increased the usefulness of this camera for studies of particulate material in the coastal zone.
A three-dimensional model, based on the coupling between a hydrodynamic part taking into account sediment dynamics (SiAM-3D) and an ecological part which accounts for the main biological compartments and processes, has been developed to study nutrient cycles (nitrogen, phosphorus, silica) and their contribution to phytoplankton stock regulation in the Bay of Seine (French coast of the English Channel). The role of each nutrient in phytoplankton production has been identified. Spring phosphorus and summer silicate limitation of diatom growth is observed and confirmed by the model. The simulations show a shift from spring nitrogen to spring phosphorus limitation of diatom growth between the 1970s and the 1990s which is related to an increase in Seine river nitrogen loading and a decrease in phosphorus. Several scenarios of nutrient input reductions were investigated and show a greater sensitivity of flagellates to these reductions. The use of an original numerical method to track the nutrient coming from different sources shows the non-negligible impact of offshore inputs in annual plankton production. In some cases, 50% of the annual diatom production could be related to open sea nitrogen loading. The annual production of flagellates supported by the open sea loading did not exceed 36%.
Nitrogen fixation is one of the important biochemical pathways that play a role in controlling the oceanic nitrogen inventory. Here we review nitrogen fixation in the ocean, with a particular emphasis on Trichodesmium, one of the dominant marine diazotrophs. Distribution data for diazotrophs are scarce, except in specific regions where Trichodesmium is known to bloom. Although some regions are clearly under-sampled, Trichodesmium can generally be found in tropical regions where temperature is at least 20 °C, except in the North Atlantic, where drift to higher latitudes is possible via the Gulf Stream. Likewise, biomass estimates are problematic because of the colony-forming habit of this organism. Trichodesmium grows slowly with reported maximum growth rates of approximately 0.14 d-1. Studies of the photosynthetic physiology indicate that Trichodesmium can tolerate high light intensity with Ik and Ic values of ∼300 and ∼140 μmole photons m−2 s−1, respectively. Review of the elemental composition of Trichodesmium indicates that the C:N molar ratio of 6.3:1 does not depart significantly from the predicted Redfield stoichiometry of 6.6:1. Overall, measured N:P ratios from the field and the laboratory were around 50, a significant departure from the Redfield stoichiometry of 16:1. Whether this indicates phosphorus limitation is not clear at present. The iron requirements of diazotrophs in general and of Trichodesmium in particular have been the subject of debate, but some recent laboratory studies have converged on Fe:C (μmole:mole) of approximately 50 at 70% of the maximum growth rates (μmax) to 250 at μmax for this species. There is a noticeable lack of information on growth rate as a function of phosphorus and fixed nitrogen sources. Although Trichodesmium is a non-heterocystous cyanobacterium, carbon and nitrogen fixation co-occur during the light period, indicating that light energy is required for both of these processes. This is likely to be achieved through cellular differentiation of the trichomes and a tight control of the temporal expression of many biochemical pathways. A summary table presents a set of values for the initial parameterisation of parameters relevant to the incorporation of nitrogen fixation in biological and biogeochemical models.
The fauna of the Snorre oil field is diverse and varies along a 40 m depth gradient across the area. Abundance and species richness increase with increasing depth but diversity is highest in the centre of the gradient. The communities present differ widely from those of the adjacent but shallower Statfjord, Gullfaks and Tordis Fields and from the major communities found throughout much of the northern North Sea, a fact attributable to the greater depth, finer sediments, higher sediment organic levels and position on the western slope of the Norwegian Trench. They have greater affinities with the communities in the Troll Field, 85 km SE on silt/clay sediments in the floor of the trench and the Heidrun Field, situated approximately 390 km N at similar depths and in an area of similar sedimentary composition. However the Snorre communities have a much higher number of suspension feeding and surface deposit feeding organisms than either of these other areas. This might be caused by higher current speeds in the Snorre area which could create favourable conditions for suspensivores. Such water movements might bring occasional incursions of intermediate Norwegian Sea water into the area. Indeed the Snorre fauna appears to bear some relationships to the intermediate depth faunas of the Norwegian Sea, and is clearly different from most of the better described northern North Sea faunas.
This paper reviews field and laboratory studies using flumes to quantify the erodability of undisturbed intertidal sediments as a function of changes in (1) the natural benthic community structure and sediment properties, and (2) the abundance of key intertidal species. Sediment erodability, which varies spatially and temporally, is dependent on the interactions between physical processes, sediment properties and biological processes, particularly the balance between two functional groups of biota, the stabilisers and the destabilisers. Bio-stabilisers can influence the hydrodynamics and provide some physical protection to the bed (e.g. mussel beds, macroalgae, salt marsh macrophytes), or can enhance cohesiveness and alter the critical erosion threshold (e.g. microphytobenthos). In contrast, bio-destabilisers (e.g. bioturbators such as Macoma balthica, Hydrobia ulvae) increase surface roughness, reduce the critical erosion threshold and enhance the erosion rate. Field studies in the Humber (England) and Westerschelde (Netherlands) have shown that interannual changes in sediment erodability were a result of a shift from a stabilised sediment dominated by microphytobenthos to a destabilised sediment dominated by M. balthica. Interannual changes in key biota, their influence on sediment erosion, and the consequences for intertidal ecology and morphology, appear to be driven in part by climatic factors (primarily a shift from mild to cold winters). Quantification and understanding of these benthic processes has been used to parameterise mathematical models of intertidal sediment dynamics, and this has provided insight into the relative importance of biological and physical factors in determining sediment erosion/accretion in the intertidal zone.
Absorption spectra (250–800 nm) of yellow substance were measured in 1993 and 1994 along five onshore–offshore transects in the Southern Bight of the North Sea in different seasons. All spectra had a common shape, so the amount of yellow substance could be established using one reference wavelength as a proxy variable. The exponential slope parameter at 380 nm (S) of the absorption spectra ranged from −0.016 to −0.023 per nm when regression was based upon the 250–650 nm wavelength range; values were 25% lower when fitting was performed over the more restricted waveband 360–540 nm, partly due to a minor shoulder at 260–275 nm in all spectra. The concentration of yellow substance, expressed as the absorption coefficient at 380 nm (a380; m−1), varied over an order of magnitude, from low values down to 0.17 m−1 in the English Channel, up to 1.00–1.25 m−1 near the Rhine and Scheldt outflow, to maxima of up to 1.75 m−1 in the neritic–estuarine waters at the entrance to the shallow Wadden Sea. This regional distribution pattern was in agreement with the known water-mass circulation and with the location of sources of yellow substance: highest concentrations close to the shore under the influence of freshwater inflow, lower values with increasing distance from the coast, where Atlantic Ocean water is the major water-mass component. No significant seasonal variation in yellow-substance concentration was found anywhere when a correction was applied for salinity. Extrapolation to zero salinity yielded an absorption coefficient at 380 nm of 5.38 m−1 for the freshwater input at both the Texel and Walcheren transects, but of 3.29 m−1 at the Noordwijk transect, reflecting differences in yellow-substance concentration between the Rhine, the Meuse, and Lake IJssel, as previously noted in fluorescence measurements. Yellow-substance concentration was not only correlated with salinity but also, albeit much less, with chlorophyll concentrations; the contribution of phytoplankton to the yellow-substance pool was marginal but significant. A relation appeared to exist between yellow-substance absorbance at 380 nm and yellow-substance fluorescence intensity; fluorescence measurements in the southern North Sea can be translated to absorption (more appropriate for ocean colour detection by remote sensing) by equating 1 mFl unit to an absorption coefficient at 380 nm of 0.056 m−1, and using the exponential relation ays(λ)=ays(λref)exp(−S(λ−λref)) for extrapolation to UV–Vis spectral absorption.
Predation by adult bivalves on bivalve larvae has been suggested to reduce larval abundance in areas with high bivalve filter-feeder biomass. Although the occurrence of larviphagy is well-studied in the laboratory, its effects in the field have scarcely been studied. We studied larviphagy at different spatial scales in the Oosterschelde estuary. On the scale of individuals, we confirmed that larviphagy occurs in Crassostrea gigas and Mytilus edulis in the Oosterschelde estuary, by examining stomach contents of adult bivalves. On a local scale, we studied effects of larviphagy by a Pacific oyster (C. gigas) bed on presence of larvae in the overlying water column by sampling larvae with fixed plankton nets. Abundance of blue mussel (M. edulis) larvae was significantly reduced by the oyster. Abundance of C. gigas larvae did not seem to be reduced by the oyster bed, but spawning by the adult oysters during the sampling period may have affected the results. On estuary-scale, the effect of larviphagy on larval abundance of C. gigas and M. edulis was studied using existing monitoring data over 6 years for M. edulis and 13 years for C. gigas. Numbers of M. edulis larvae showed no significant trend over the 6 years studied. Abundance of C. gigas larvae declined with an increasing filter feeder stock (that was mainly caused by an increase in C. gigas stock). This decline may be due to direct effects of larviphagy or indirect effects such as lowered food levels, and was not compensated by an increased larval production. All results combined, complemented with a theoretical estimate of the effect of larviphagy on estuary-scale, strongly suggest that larviphagy is major source of mortality for bivalve larvae in the Oosterschelde estuary.
The aim of the present study was to analyse the influence of warming on flatfish populations in the Bay of Biscay. 17 autumn cruises conducted from 1987 to 2006 over the whole shelf of the Bay of Biscay provided data for the abundance and occurrence of adults for twenty flatfish species. Trends in flatfish abundance were analysed with regard to geographic range of populations and interannual fluctuations in abundance were related to seawater temperature.Results showed significant trends in abundance and occurrence for 55% of the flatfish species in the Bay of Biscay. The response to warming of seawater was correlated to geographic ranges of species. While the abundance of the northern temperate species decreased, that of southern ones increased. Moreover, for 40% of the species which densities have significantly changed, abundances were correlated to temperatures in their year of birth, positively for southern species and negatively for northern ones. Last, the abundance of flatfish adults over the Bay of Biscay was compared to previous data on juveniles in the Bay of Vilaine, one of the estuarine nursery ground in this area. For the northern species which have disappeared, the decline in juvenile abundances preceded that of adults by several years, indicating that the recruitment is the process affected.We concluded on a major impact of warming to explain changes in flatfish species abundances. Nevertheless, the impact of fishing interacts with that of climate change because the exploited species appeared to be the most negatively affected.
Line-transect data from sighting surveys conducted in the western Mediterranean (in 1991) and the Alboran Sea (in 1992) were analysed to estimate densities and numbers of striped and common dolphins in various areas of the western Mediterranean. Density of striped dolphins in the northwestern Mediterranean was estimated as 0.20 dolphins km−2 (CV = 0.24; 95% CI = 0.12 and 0.32) and was 41% higher than in the southwestern Mediterranean, where it was estimated as 0.12 dolphins km−2 (CV = 0.38; 95% CI = 0.05 and 0.25). The highest densities were observed in the Liguro–Provençal basin, with 0.24 dolphins km−2 (CV = 0.26; 95% CI = 0.14 and 0.40), and the Alboran Sea, with 0.20 dolphins km−2 (CV = 0.33; 95% CI = 0.10 and 0.36). These areas, and especially the Ligurian Sea, appear to be the most productive in terms of the food consumed by striped dolphins. Common dolphins were abundant only in the Alboran Sea with an estimated density of 0.16 dolphins km−2 (CV = 0.40; 95% CI = 0.08 and 0.35), scarce in the south Balearic area and almost absent in the northwestern Mediterranean. The magnitude of the dolphin by-catch in fishing operations in the Alboran Sea and other areas stresses the need for further assessment of densities and numbers, notably in the Alboran Sea and the North African Mediterranean waters.
We address the question of whether year-to-year variability in pelagic algal food supply can explain long-term variability in macrozoobenthic biomass in an estuarine area. Starting in the early 1970s, quantitative data were frequently collected in standardized ways in the western part of the Dutch Wadden Sea on (1) concentrations of phytoplankton species and chlorophyll (and rates of primary production) in the main tidal inlet (Marsdiep) and (2) numerical densities and biomass of macrozoobenthic animals (and growth rates in a few species) in a nearby extensive tidal-flat area (Balgzand). In both data series, the most distinctive feature was a sudden change that took place around 1980, viz. a rather sudden and persisting doubling of concentrations of chlorophyll and algal cells and of primary production rates, as well as of numerical densities and biomass of zoobenthos. From these parallel changes we hypothesise that algal food largely determines the abundance of zoobenthos in the Wadden Sea. The following observations substantiate this hypothesis: (1) the significant correlation between annual mean values of chlorophyll concentration and overall mean numerical density and biomass of zoobenthos (as estimated after an appropriate time lag), (2) the observed limitation of zoobenthic biomass doubling (after the doubling of food supply) to areas with already high biomass values (where food demand was high and food could therefore be in short supply), (3) the limitation of a strong response to changes in food supply to functional groups that are directly dependent on algal food, i.e. suspension and deposit feeders, as opposed to carnivores, (4) the significant correlation between annual growth rates in Macoma balthica and food supply in the growing season, particularly in areas close to the tidal inlet where food concentrations were monitored.
Egg and larval abundances for Northeast Arctic cod estimated annually from 1959–1993 were combined with estimates of spawning stock biomass (SSB) and stage abundance estimates for the larvae, pelagic juveniles and demersal juveniles to determine when a correlation with recruitment could first be detected and to track how the correlation changed with increasing age/stage. The largest increase in explanatory power occurred going from SSB to the egg stage suggesting that the transition from potential to realised egg abundance is a critical stage and/or that SSB is a poor index of the potential total egg production. Egg stage abundance was significantly correlated with recruitment; however, the portion of the recruitment signal explained by this index differed from the portion explained by subsequent stages suggesting that the signal in year-class strength undergoes modification between the egg and larval stages. Contrary to the expectation that the signal in year-class strength should become stronger as the penultimate recruitment stage is approached, the significance of the correlations with recruitment decreased for the larval and juvenile stages relative to the egg stage suggesting that the measurement error for these latter stages is high. Temperatures during the post-settlement period explained a significant portion of the residual variation in stage abundance/recruitment relationships for the larval, pelagic juvenile and demersal juvenile stages. A multivariate recruitment model was therefore developed and tested using independent data for the periods preceding and following the model calibration period.
Samples taken regularly from the intake screens of West Thurrock power station between January 1977 and November 1992 were used to investigate the factors controlling flatfish abundance (flounder: Platichthys flesus; dab: Limanda limanda; plaice: Pleuronectes platessa; sole: Solea solea) in the middle Thames estuary. Most sampled fish were age-0. All species followed regular patterns of seasonal occurrence, had distinctive seasonal abundance peaks and demonstrated only minimal temporal overlap. Multiple-regression models were used to study the relationship between physico-chemical variables, physico-chemical variable interactions, trends and seasonal factors on fluctuations in flatfish sample abundance. Flounder abundance was significantly influenced by temperature, suspended solids, oxygen-flow and oxygen-temperature interactions, trend and seasonal variables. Dab abundance was significantly related to suspended solids, oxygen-flow, oxygen-temperature and flow-chlorinity interactions, Crangon crangon abundance and seasonal variables. Sole abundance was controlled only by seasonal variables, whilst plaice abundance could not be adequately explained by the available physico-chemical data. The combination of variables affecting flatfish abundance indicates species using the estuary were affected by a complex set of events within the estuary. The interactions between routinely measured environmental variables further suggest that estuarine monitoring studies must attempt to understand the nature of possible interactions between variables if an improved understanding of the changes in estuarine fish communities resulting from human activity is to be gained.
Spring distribution and abundance of lesser sandeels during the day were linked to zooplankton densities, seabed substrate and various hydrographic factors using small scale empirical data collected in two areas on the Dogger Bank in 2004, 2005 and 2006. The results of a two-step generalized additive model (GAM) suggested that suitable seabed substrate and temperature best explain sandeel distribution (presence/absence) and that sandeel abundance (given presence) was best described by a model that included bottom temperature, difference between surface and bottom temperature and surface salinity. The current study suggests that suitable seabed substrate explains sandeel distribution in the water column. Bottom temperature and surface salinity also played an important role in explaining distribution and abundance, and we speculate that sandeels favour hydrographically dynamic areas. Contrary to our hypothesis sandeels were not strongly associated with areas of high zooplankton density. We speculate that in early spring on the western Dogger Bank plankton is still patchily distributed and that sandeels only emerge from the seabed when feeding conditions near their night-time burrowing habitat are optimal. The results also suggested that when abundance is over a threshold level, the number of sandeel schools increased rather than the schools becoming bigger. This relationship between patchiness and abundance has implications for mortality rates and hence fisheries management.
The vertical and temporal distribution of two calycophoran siphonophores, Chelophyes appendiculata (Eschscholtz, 1829) and Abylopsis tetragona (Otto, 1823) in the Bay of Villefranche (northwestern Mediterranean) was investigated by an analysis of three different planktonic time series. A daily series (1993–1995) showed seasonal peaks of the nectophores of C. appendiculata during spring and particularly in late summer, while the abundance of A. tetragona remained similar throughout the year. A weekly series (1994–1995) showed that C. appendiculata (nectophores and eudoxids) became concentrated above the thermal discontinuity, in the most stratified and warm waters, whereas A. tetragona was collected in large numbers below this discontinuity. A 27-year survey (1966–1993) showed long-term fluctuations of these siphonophore populations, which became abundant in the Bay starting from 1980 and especially after 1984, when the water column grew warm and hypersaline, corresponding to a less rainy period. Temporal (seasonal and long-term) and bathymetric (between 10 and 60 m depth) successions of these two siphonophores were noted in this shallow coastal bay.
We have assessed the impact of seasonal pulses of phyto-detritus towards the bottom of the Porcupine Abyssal Plain (PAP) in the NE Atlantic. Phytopigments were determined (HPLC) in sediment, near-bottom suspended matter and sediment trap samples to obtain an impression of the quantity and quality of the organic material reaching the bottom. Its supposed effect on the benthic community was estimated on the basis of in situ Sediment Community Oxygen Consumption (SCOC) rates, deck oxygen micro profiles as well as chlorophyll mixing and degradation rates. There was considerable interannual variation in the deposition of fresh organic material. In September 1996 the mass flux at 10 m above the bottom was 0.25 g DW m−2 d−1. The material caught in the trap was relatively fresh with a chlorophyll-a : phaeophorbide ratio of 1.33. During the other seasons (March 1997, July 1997 and October 1997) such fresh material was absent and even in July, the period in which fresh material is supposed to have arrived on the bottom, the ratio remained low, viz. 0.23. Based on these findings we conclude that in 1997 only small amounts of already degraded material arrived at the sea floor. On the basis of the large quantities of fresh material found in September 1996, the existence and subsequent deposition of autumn blooms should be considered.With in situ respirometry no effect of the differential quality and quantity of the settling detritus could be detected, whereas model parameters based on oxygen microprofiles demonstrated changes associated with the supposed seasonal cycle. Also the downcore pigment distribution of chlorophyll-a and derived mixing and degradation coefficients showed variations which were linked to the observed seasonal changes in input of organic matter.
In the Marsdiep tidal inlet, Phaeocystis globosa blooms have occurred every year since phytoplankton monitoring started in the early 1970s. A high spring peak is usually followed by lower peaks in summer. Detailed cell counts are presented for the years 1993, 1994, and 1995, of which 1994 was exceptional with a low spring peak but a much higher peak in June/July. Over tidal flats in the western Dutch Wadden Sea, I found Phaeocystis colony accumulation and cell densities one to two orders of magnitude higher than in the Marsdiep tidal inlet; colonies settled on the tidal flats, but did not become part of larger aggregates. The accumulation process resembles similar processes earlier described for fine sediments in the Wadden Sea and may be caused by similar processes. The small benthic gastropod Hydrobia ulvae at low tide grazed on colonies settled on the sediment. This loss of colonies from the water column by settling and grazing may help in rapidly decreasing P. globosa cell numbers after the spring peak in the Wadden Sea. Massive sedimentation and subsequent mineralization of Phaeocystis colonies might induce low oxygen events and mass mortality of macrobenthos, but up to now no direct relation has been reported between Phaeocystis sedimentation and mass mortality in zoobenthos.
The beaches of the North Sea barrier island Schiermonnikoog (The Netherlands) are covered by microbial mats. Five types of microbial mats were distinguished based on a variety of characteristics, located along a transect perpendicular to the coast. Biomass abundance and composition of the microbial community were analyzed in these mats using pigments and phospholipid-derived fatty acids (PLFA) as biomarkers. Biomass per gram sediment increased more than six-fold from the mats at the low water mark to mats found at the edge of the dunes. Microscopic analysis revealed that the increase in biomass was accompanied by a change in species composition. Pigment- and PLFA composition reflected the changes in species composition. The PLFA data could be used to estimate the relative group abundance using the matrix factorization program CHEMTAX., whereas the pigment data were found not to be suitable for this purpose.
Particulate amino acids (THAA) were measured in the East Frisian Wadden Sea at monthly intervals from February to December 1995. Each time two tidal cycles were sampled at hourly intervals. THAA made up between 3 and 50% (mean 15±7%) of the particulate organic carbon and between 9.3 and 104% (mean 49±20%) of the total nitrogen pools. Seasonal variations of THAA followed the development of phytoplankton in the water column while tidal variability was controlled largely by physical forcing such as tidal current velocities and wave action. The overall molar composition of the THAA pool appears to be dominated by a detrital signal originating from erosion of surface sediments.
The fate of a Phaeocystis globosa bloom in the southern North Sea off Belgium, the Netherlands and Germany in May 1995 was investigated during a cruise with RV ‘Belgica’. We used fatty acids as biomarkers to follow the fate of Phaeocystis-derived biomass of a Phaeocystis-dominated spring bloom. The bloom, in which up to >99% of the biomass was contributed by Phaeocystis, showed a fatty acid composition with a characteristically high abundance of polyunsaturated C18-fatty acids, which increased in concentration with number of double bonds up to 18:5 (n-3), and high concentrations of 20:5 (n-3) and 22:6 (n-3). In contrast to most previous studies, fatty acid analysis of the mesozooplankton community (mainly calanoid copepods) and meroplankton (Carcinus maenas megalope) indicated that P. globosa was a major component (ca. 70% and 50%, respectively) in the diet of these organisms. Massive accumulations of amorphous grey aggregates, in which Phaeocystis colonies were major components, were dominated by saturated fatty acids and contained only few of the polyunsaturated C18-fatty acids. A hydrophobic surface slick that covered the water surface during the bloom showed very similar patterns. Foam patches contained few Phaeocystis-typical fatty acids, but increased amounts of diatom-typical compounds such as 16:1 (n-7) and 20:5 (n-3), and 38% fatty alcohols, indicating that wax esters dominated the lipid fraction in the foam with ca. 76% (w/w). The fatty acid compositions of surface sediment showed that no sedimentation of fresh Phaeocystis occurred during the study. The results indicate that Phaeocystis-derived organic matter degraded while floating or in suspension, and had not reached the sediment in substantial amounts.
In February, May and August 1994, four stations in the North Sea (viz. at the Broad Fourteens, Frisian Front, German Bight and Skagerrak) were visited to sample near-bottom particulate organic matter. Samples, taken by means of a pump, a sediment trap and a sediment recorder, were analysed on organic carbon, total nitrogen, phytopigments and fatty acids. These molecular markers were used to describe the nature and quality of the organic particles in the near-bottom water. Principal component analysis showed chlorophyll a, phaeopigments and fatty acids to be useful markers for the quality of organic matter and yield complementary information.The quality of the near-bottom particles appeared to be related to the local hydrography and depositional circumstances. The Broad Fourteens station, a non-depositional sandy site along the Dutch coast, showed organic particles to be relatively fresh, little influenced by resuspended sedimentary material. Near-bottom organic particles on this site contained relatively high shares of chlorophyll a and polyunsaturated fatty acids, characteristic of algal matter. On the other hand the particulate organic material on the two depositional locations, the Frisian Front and the German Bight stations, was influenced by resuspension of sedimentary organic particles poor in pigments and fatty acids. Amounts of carbon trapped in the near-bottom environment at the Skagerrak station were lower than expected from the literature.
Flower and fruit production of the abundant, tall, long-lived, dioecious, surface-pollinating seagrass species Enhalus acoroides (L.) Royle were estimated at seven sites in the reef flats off Bolinao (NW Luzon, The Philippines) featuring different fragmentation of the seagrass meadows. Fragmentation of the seagrass meadow was quantified as cover of E. acoroides and all seagrass species present in 20×20 m plots. E. acoroides and overall seagrass cover were correlated positively. The proportion of female flowers of E. acoroides that developed a fruit increased sharply as overall seagrass cover was around 50%. Apparent sex ratio bore no relationship with overall seagrass cover. This threshold-type of relationship suggests that fragmentation of seagrass meadows can have a major effect on the reproductive output of this species. A possible mechanism underlying these results would be a non-linear increase of the efficiency of trapping the surface-dispersed pollen with increasing seagrass canopy density. This provides the first evidence based on real data that fragmentation can affect the population dynamics of seagrass species.
Density, biomass and community structure of macrofauna were estimated together with several sediment characteristics at seven stations ranging from 208 m to 4460 m water depth along the OMEX transect in the Goban Spur area (NE Atlantic) during three seasons (October 1993, May 1994, and August 1995). Median grain size decreased with increasing water depth and showed no differences between the seasons. The percentages of organic carbon and total nitrogen were highest at mid-slope depths (∼1000 to 1500 m), and were significantly higher in August at the upper part of the slope to a depth of ∼1500 m. The C:N ratio in the surface layer amounted to ∼7 to 8 in May, ∼10 to 12 in August and ∼14 to 17 in October at all stations (except the deepest at 4460 m, where it was ∼11 in May and August), indicating arrival of fresh phytodetritus in May, and therefore seasonality in food input to the benthos. Densities of macrofauna decreased exponentially with increasing water depth. Significantly higher densities of macrofauna were found in May at the upper part of the slope to a depth of ∼1500 m. These differences were mainly due to high numbers of postlarvae of echinoids at the shallowest station and ophiuroids at the deeper stations. Biomass values also decreased with increasing water depth, but biomass was relatively high at the ∼1000 m station and low at ∼1500 m, due to relatively high and low mean weights of the individual macrofaunal specimens. No significant differences in biomass were found between the seasons. Respiration was high (15 to 20 mgC·m−2·d−1) in May at the upper part of the slope to a depth of ∼1000 m and low (1–3 mg C·m−2·d−1) at the deeper part. At the shallowest stations to a depth of ∼1000 m respiration was highest in May, at the mid-slope stations (∼1400–2200 m) it was highest in August, whereas the deepest stations (∼3600 to 4500 m) did not show any differences in respiration rates. In conclusion; seasonal variation in organic input is reflected in denstiy, community structure and activity of the macrofauna along the continental slope in the NE Atlantic.
Different estimates were used to assess the diversity of the total macrofauna and its major taxonomic groups separately from a broad bathymetric range at a site in the NE Atlantic. In the Goban Spur region, a transect was sampled from the shelf to the abyssal plain over a depth range from ∼200 to ∼4500 m and in the Porcupine Sea Bight two stations were sampled (at 3670 m and 4115 m). Species diversity (the number of species per number of individuals) increased with increasing water depth, both when expressed as Hurlbert's E(Sn) and as Shannon's . The expected number of species in a 100-individual sample E(S100) of total macrofauna increased from 30 on the shelf to 68 on the abyssal plain. Evenness (the proportional abundance of species), estimated with Shannon's J′, also increased with water depth from 0.66 to 0.91, whereas dominance (Simpson's D) decreased from 0.09 to 0.01. Species richness (the number of species per unit of area), however, showed a parabolic pattern with a peak at the upper slope. The largest number of species was found at the slope station at 1425 m (232 species within 0.66 m2). It is argued that species richness is not a synonym of species diversity, but that species richness depends both on species density (which decreases with increasing water depth) and on species diversity. Across the whole bathymetric range (∼200 to ∼4500 m) a total of 696 species within 8327 specimens in a total sampled area of 4.12 m2 were counted, yielding mean values of ∼12 individuals per species and ∼169 species per m2. Different communities were found to exist on the shelf, slope and abyss. It is suggested that this could have been caused by different selection processes. Differences in life-history strategies and organic-matter supply could (at least partly) explain the different community structures and diversity patterns found along the depth gradient.
The flagellate micro-alga Fibrocapsa japonica can form harmful algal blooms along all temperate coastal regions of the world. The species was first observed in coastal waters of Japan and the western US in the 1970s; it has been reported regularly worldwide since. To unravel whether this apparent range expansion can be tracked, we assessed genetic variation among nuclear ribosomal DNA ITS sequences, obtained from sixteen global strains collected over the course of three decades. Ten sequence positions showed polymorphism across the strains. Nine out of these revealed ambiguities in several or most sequences sampled. The oldest strain collected (LB-2161) was the only one without such intra-individual polymorphism. In the others, the proportion of ambiguities at variable sites increased with more recent collection date. The pattern does not result from loss of variation due to sexual reproduction and random drift in culture because sister cultures CS-332 and NIES-136 showed virtually the same ITS-pattern after seven years of separation. Neither are the patterns explained by recent range expansion of a single genotype, because in that case one would expect lowest genetic diversity in the recently invaded North Sea; instead, polymorphism is highest there. Recent ballast-water-mediated mixing of formerly isolated populations and subsequent ongoing sexual reproduction among them can explain the increase in ambiguities. The species’ capacity to form harmful blooms may well have been enhanced through increased genetic diversity of regional populations.
The influence of macrofaunal reworking activities on the redistribution of particle associated hydrocarbon compounds (HC) was experimentally investigated. Two distinct hydrocarbon mixtures adsorbed on montmorillonite particles (< 4 μm diameter) were added to the surface and deeper (2.5 cm) sediment layers. For comparison, luminophores (100–160 μm diameter) were added in the two deposit layers. At the start of the experiment, four macrobenthic species (the bivalve Abra nitida, the polychaete Scalibregma inflatum, and the echinoderms Amphiura filiformis and Echinocardium cordatum) were added to the sediment surface. The macrofauna added rapidly transferred HC from the surface sediment down to ∼5 cm depth by both continuous (biodiffusion) and non-continuous (biotransport) transport. Hydrocarbon compounds initially added to the deeper sediment layer were only subject to biodiffusion-like transport. Apparent biodiffusion coefficients (Db) quantified by using a 1-D model were between 0.5 and 8.4×10−3 cm 2 d−1, and biotransport coefficients (r) ranged from 2.0 to 27.6×10−3 d−1. Thus, the four species studied did not have the same effect on particle redistribution and, consequently, on HC repartition in the sediments. E. cordatum was the most efficient reworker. The present study demonstrated the importance of particle size selectivity by benthic fauna, and verified that macrofaunal reworking activities may redeposit sediment from deeper sediment layers on the sediment surface. Both processes have obvious implications for rates and pathways during organic matter mineralisation in marine sediments.
The nursery habitats of flatfishes range from offshore areas to rivers, with salinity conditions varying significantly between the habitats selected by larvae and juveniles. Morphological characteristics of larvae also contribute to the successful recruitment to specific nursery areas. In the present study, ontogenetic developments of low-salinity tolerance and morphology of five pleuronectid flatfishes were examined, and compared with their previously described ontogenetic migrations. Species examined were starry flounder Platichthys stellatus, stone flounder Platichthys bicoloratus, spotted halibut Verasper variegatus, marbled flounder Pseudopleuronectes yokohamae and slime flounder Microstomus achne. The first three were categorised as ‘cross-shelf species’, marbled flounder as ‘demersal-egg species’ and slime flounder as ‘offshore species’. Their typical nurseries were rivers, estuaries, tidal flats, coastal areas and offshore areas, respectively. Low-salinity tolerance from hatching to juvenile stage was examined by survival 48 h after transfer (A to I stage) and seven days after transfer (I stage) from seawater to various salinities (0, 1, 2, 4, 8, 16 and 32 ppt). High survival in 4 ppt was found in all yolk-sac larvae, but decreased in mid-larval development. During metamorphosis, starry flounder, stone flounder and spotted halibut developed strong low-salinity tolerance and juveniles were able to survive in 0, 1 and 2 ppt, respectively. In contrast, marbled flounder and slime flounder did not show these clear developments and juveniles could only survive in 4 and 8 ppt, respectively. In the morphological development, differences in standard length (SL) and relative body depth (BD) to SL (%BD/SL, an indicator of locomotive adaptation of flatfish larvae showing tilt swimming behaviour during the eye translocation) increased with larval development, and the greatest SL and %BD/SL were observed at the metamorphic climax H stage in the order of slime flounder (24.0 mm, 61.7% at 110 days after hatching, DAH), spotted halibut (14.5 mm, 42.1% at 50 DAH), stone flounder (9.5 mm, 41.1% at 34 DAH), starry flounder (7.0 mm, 34.8% at 28 DAH) and marbled flounder (6.9 mm, 28.5% at 24 DAH). ‘Cross-shelf species’ are characterised by strong inshore migration and settlement in shallower nurseries, ‘demersal-egg species' by a small habitat shift from coastal spawning to nursery areas with faster settlement of larvae with small size and slender shape, and ‘offshore species’ are characterised by gradual settlement in deep areas by locomotive adaptations of larvae with a large body surface area. So the development of low-salinity tolerance and morphology closely reflect the known ontogenetic migrations of the five species along the nearshore-offshore gradients. Especially, inverse relationships between low-salinity tolerance and metamorphic size in the three cross-shelf species, together with the general patterns of lower predator densities in lower salinity environments, suggest that low-salinity adaptability is critical for the survival strategy of species using shallow, nearshore areas as nurseries.
Sea level and atmospheric parameters are estimated using an upward-looking 600 kHz acoustic Doppler current profiler (ADCP) at ranges close to the sea surface, known to be unsuitable for measuring ocean currents. The ADCP is moored at the bottom, 45 m below the surface in the central North Sea under occasionally severe winter conditions with significant wave heights (Hs) up to 12 m and wind speeds exceeding 20 m s−1. ADCP estimates of sea level (relative accuracy <0.02 m) reveal that a bottom pressure recorder shows an artificial datum depression down to about — 0.03Hs m due to wave action just above the bottom. The proper determination of the sea surface location turned out to be crucial for ADCP estimates of waves and winds.Significant wave height is estimated better than 0.7 m using ADCP's back-scattered amplitude (‘echo intensity’) signal from depths a few metres below where the acoustic signal directly hits the surface. At these depths, echo intensity is related to air bubble amounts, proportional to Hs. In contrast, echo intensity observed close to and at the surface is inversely proportional to Hs, due to wave smearing. It is demonstrated that high-frequency velocity variance (noise) in ADCP current data is not instrumental but predominantly induced by surface wave aliasing. It is suggested that this variance be used to infer significant wave number and frequency.Wind stress is estimated after subtracting mid-depth ADCP current data from those obtained close to the surface, in a surface following co-ordinate system. Wind stress magnitude is estimated better than 0.1 Pa (with values exceeding 1 Pa) and wind direction better than 10°. It is concluded that the ADCP ‘measures’ the downwind component related to Langmuir circulation, rather than Ekman surface currents.
A unique, five-year long data set of ferry-mounted ADCP measurements in the Marsdiep inlet, the Netherlands, obtained between 1998 and 2003, is presented. A least-squares harmonic analysis was applied to the water transport, (depth-averaged) currents, and water level to study the contribution of the tides. With 144 tidal constituents, maximally 98% of the variance in the water transport and streamwise currents is explained by the tides, whereas for the stream-normal currents this is maximally 50%. The most important constituent is the semi-diurnal M2 constituent, which is modulated by the second-largest S2 constituent (about 27% of M2). Compound and overtides, such as 2MS2, 2MN2, M4, and M6, are important in the inlet. Due to interaction of M2 with its quarter-diurnal overtide M4, the tidal asymmetry in the southern two thirds of the inlet is flood dominant. The amplitudes of all non-astronomic constituents are largest during spring tides, strongly distorting the water level and velocity curves. The M2 water transport is 40° ahead in phase compared to the M2 water level, reflecting the progressive character of the tidal wave in the inlet. The currents are strongly rectilinear and they are sheared vertically and horizontally, with the highest currents at the surface above the deepest part of the inlet. During spring tides, near-surface currents can be as large as 1.8 m s− 1. Due to the relative importance of inertia compared to friction, the M2 currents near the centre (surface) lag maximal 20° (3°) in phase with the currents near the sides (bottom). The tidal-mean currents are directed into the basin in the shallower channel to the south and out of the basin in the deeper channel to the north.
In this study we modelled the occurrence of the shorebirds in their intertidal feeding areas using Generalised Additive Models (GAMs). The data used for the modelling exercise consisted of regular winter counts of shorebirds in one hundred and five 50×50 m plots, arranged in a 1050×250 m area. Several physical characteristics were obtained from each plot and some other variables were derived using a Geographical Information System (GIS). These variables were used to predict the probability of occurrence of shorebirds using GAMs. The three most influential variables in the distribution of the shorebird species assemblage were the distance to channels, type of sediment and area of oyster beds. The smooth response curves of the species occurrence along the gradient of the variables were biologically meaningful, and generally consistent with previous descriptions of the habitat preferences of each species. The quality of the fits was generally very high, as assessed by the significance of the models and by comparing the observed frequencies with the predicted probabilities of occurrence. The performance of GAMs was compared with that of the equivalent GLMs, and we concluded that extended flexibility offered by GAMs resulted in better overall fits. We suggest that GAMs represent a convenient framework for modelling the large-scale distribution of shorebirds in intertidal areas from their physical characteristics.
Many flatfish species are caught in mixed demersal trawl fisheries and managed by Total Allowable Catch (TAC). Despite decades of fisheries management, several major stocks are severely depleted. Using the Common Fisheries Policy (CFP) as an example, the failure of mixed-fisheries management is analysed by focussing on: the management system; the role of science; the role of managers and politicians; the response of fisheries to management. Failure of the CFP management could be ascribed to: incorrect management advice owing to bias in stock assessments; the tendency of politicians to set the TAC well above the recommended level; and non-compliance of the fisheries with the management regulations. We conclude that TAC management, although apparently successful in some single-species fisheries, inevitably leads to unsustainable exploitation of stocks caught in mixed demersal fisheries as it promotes discarding of over-quota catch and misreporting of catches, thereby corrupting the basis of the scientific advice and increasing the risk of stock collapse. This failure in mixed demersal fisheries has resulted in the loss of credibility of both scientists and managers, and has undermined the support of fishermen for management regulations. An approach is developed to convert the TAC system into a system that controls the total allowable effort (TAE). The approach takes account of the differences in catch efficiency between fleets as well as seasonal changes in the distribution of the target species and can also be applied in the recovery plans for rebuilding specific components of the demersal fish community, such as plaice, cod and hake.
This study aims to study the diversity of the migration patterns of the anchovy Engraulis encrasicolus (L., 1758) collected in the Guadiana estuary (SE-Portugal/SW-Spain) and in the adjacent coastal area, by analysing Sr concentration distribution in sagittal otoliths. The distribution of Sr in otoliths of anchovy captured in the estuary allowed us to identify two major patterns: a) fish that do not migrate through estuarine salinity gradients and b) fish that make differential use of the estuary throughout their lives, occupying areas of lower salinity during the early stages of the life cycle followed by migrations to areas of higher salinity later in life. The patterns for fish captured in coastal areas inferred that they probably hatched in an estuarine environment and progressively migrated along an estuary to the coast. There is no massive migration of coastal anchovy to spawn in the estuary, as previously suggested. These results suggest that fishery managers should evaluate the contribution of estuarine populations to coastal stocks and the impact of estuarine recruitment fluctuations on this stock.
Estuaries are regarded as valuable nursery habitats for many commercially important marine fishes, potentially providing a thermal resource, refuge from predators and a source of abundant prey. To assess the extent of estuarine use by juvenile (0+) common sole (Solea solea), whiting (Merlangius merlangus) and European seabass (Dicentrarchus labrax) we: (1) developed techniques to distinguish between estuarine and coastally-caught juveniles using otolith chemistry; and (2) examined the accuracy with which multi-elemental signatures could re-classify juveniles to their region of collection.High-resolution solution-based inductively coupled plasma mass spectrometry (HB-SB-ICPMS) was used to quantify 32 elements within the juvenile otoliths; 14 elements occurred above detection limits for all samples. Some elemental distributions demonstrated clear differences between estuarine and coastally-caught fish. Multivariate analysis of the otolith chemistry data resulted in 95–100% re-classification accuracy to the region of collection. Estuarine and coastal signatures were most clearly defined for sole which, compared to bass and whiting, have low mobility and are less likely to move from estuarine to coastal habitats between larval settlement and later migration to adult stocks. Sole were the only species to reveal an energetic benefit associated with an estuarine juvenile phase. The physiological ability of bass to access upper estuarine regions was consistent with some elemental data, while the high mobility and restricted range of whiting resulted in less distinct otolith chemistries.
The spatial and temporal distribution of dimethylsulfide (DMS) was investigated in surface seawater and in the marine atmosphere in the Pearl River Estuary and northern South China Sea during three cruises in July 2000, May 2001 and November 2002. Sea-to-air fluxes of DMS were subsequently estimated based upon seawater DMS concentration, temperature of surface seawater and wind speed over sea. The seawater DMS concentration of the three cruises ranged from 0.1 to 52.7 nmol l−1 (n=76). DMS concentrations showed remarkable spatial and temporal distributions and highest values were observed at the mouth of the Pearl River Estuary. Throughout the study area we observed high levels of DMS in the water with great sea-to-air flux and relatively low levels of atmospheric DMS (1.70±1.16 nmol m−3 in May 2001 and 2.25±0.38 nmol m−3 in November 2002). Aerosol components, potentially linked with DMS oxidation, were also measured. The atmospheric concentrations of nss-sulfate and nitrate were much higher in the Pearl River Estuary than in the offshore area, with mean values of 12.11 and 4.45 μg m−3 for nss-sulfate, 4.88 and 2.21 μg m−3 for nitrate. Aerosol mass and components' concentrations decreased from the inner estuary to outer waters. High concentrations of nss-sulfate and nitrate in sea salt particles imply that oxidation of atmospheric DMS is related with anthropogenic sources and heavy ozone, NOx and SO2 pollution in the study area.
Benthic activity was examined at three stations (18 m water depth) in the northwestern Adriatic Sea. Carbon mineralisation rates, as based on sediment oxygen consumption rates, ranged from 54 to 89 g C m−2 y−1. The relatively high carbon mineralisation rates, large macrofaunal biomass (9 to 16 g C m−2) and macrofaunal production (11 to 19 g C m−2 y−1) provide evidence of high organic-matter input and intense benthic–pelagic coupling. This is further supported by the high dominance of the suspension-feeding bivalve Corbula gibba, which accounts for 52 to 63% of the total annual macrofaunal biomass production. Although the infaunal distribution of total macrofauna showed a sharp decline in densities and biomass with depth into the sediment, different patterns within the dominant taxa were observed. Whilst the bivalve Corbula gibba and the amphipod Ampelisca sp. were restricted to the surface layer, other species such as the dominant bivalve Mysella sp. and the gastropod Hyala sp. were not confined to a specific depth level and the majority of the populations occurred deeper than 5 cm into the sediment. Bioturbation, based on the occurrence of macrofauna, extended to at least 20 cm. Nematodes and foraminifera together formed 80 to 90% of the meiofaunal community in the upper 5 cm of the sediment. Annual mean densities ranged from 3.40 to 6.07×106 ind. m−2. Maximum abundance of meiofauna was not encountered at the station where maximum macrofaunal activity was recorded, and this could reflect the negative effect of biological interaction on meiofaunal densities in areas that have a high food supply.
Top-cited authors
Henk W. van der Veer
  • NIOZ Royal Netherlands Institute for Sea Research
Adriaan D. Rijnsdorp
  • Wageningen University & Research
Jaap van der Meer
  • Wageningen University & Research
Jacqueline Stefels
  • University of Groningen
Olav Sigurd Kjesbu
  • Institute of Marine Research in Norway