A seasonal study of the distributions of surface state variables in Liverpool Bay. III. An offshore front

Variously orientated transects through Liverpool Bay in December, 1976, revealed a narrow but steep density discontinuity running north to south through the surface waters. Surface distribution maps from cruises during the winter of 1976–1977 showed that the density discontinuity provided a sharp line of demarcation between the physical and chemical characteristics of the offshore and coastal waters. Over the winter, an accumulation of river discharged nutrients in the waters bounded by the front and the coast generated a large nutrient reservoir in the inshore waters and propagated a difference in the nutrient availability ratios across the front. As a consequence, by March the inshore waters were characterized by lower salinities and temperatures, higher dissolved inorganic nutrient concentrations and lower silicon to total nitrogen ratios than the offshore waters. Prior to the 1977 spring phytoplankton bloom in Liverpool Bay, these two contrasting water types were in juxtaposition along the length of the front.     

A modelling investigation of the distribution of stratification and phytoplankton abundance in the Irish Sea

Thermal stratification and phytoplankton abundance are modelledon a 5 km grid covering the Irish Sea. The water column is approximatedby three layers. The top layer is uniformly mixed by wind stirringand the bottom by tidal energy, while linear gradients can occurin the middle layer. The model is forced with hourly meteorologicaldata and mean tidal energies. Primary production is representedby a model with a single nutrient and a single phytoplanktonpopulation. The results from the model show good agreement withdata collected on a Ministry of Agriculture, Fisheries and Food(MAFF) cruise in May 1992 and with historical data. When advectionis included, driven by depth-averaged currents, the surfacetemperature patterns are improved but bottom temperatures indeep water are raised and high concentrations of chlorophyllare carried offshore from coastal regions. This indicates alimitation of using depth-averaged currents and a need to accountfor differences in phytoplankton species composition in coastaland offshore waters. Calculations demonstrate the importanceof salinity variations to stratification and phytoplankton growth.Smoothing the wind mixing energy has the effect of delayingthe onset of the spring bloom in areas where wind mixing issignificant. Removing the diurnal cycle of solar heating alsodelays the spring bloom. The chlorophyll gradient in the middlelayer has a large impact on the response of the model to short-termvariability in the meteorological forcings.     

Are copepods important grazers of the spring phytoplankton bloom in the western Irish Sea?

The spring phytoplankton bloom and copepod grazing were studied at a coastal and offshore station in the western Irish Sea during 1997. Maximum chlorophyll standing stocks of 132.8 mg m^-2 inshore and 199.4 mg m^-2 offshore were measured in late April. At that time, mean water column temperatures were 10 and 8C at the coastal and offshore station, respectively. Spring bloom production at the coastal station was estimated as 31.2 g C m^-2 and was dominated by the diatom Guinardia delicatula. Offshore, production was 28.2 g C m^-2 and the bloom was composed of small (10 m) phytoflagellates and the silicoflagellate Dictyocha speculum. Maximum copepod abundance (189 and 544 x 10³ individuals m^-2, inshore and offshore, respectively) coincided with the spring bloom. Pseudocalanus and Temora ingestion rates were derived from measurements of gut pigment fluorescence, and were found to vary during the course of the spring bloom as a result of changes in gut content. Grazing by late copepodite and adult Pseudocalanus and Temora was variable inshore, but overall accounted for 17% of bloom production. Offshore, 22% of bloom production was grazed with maximum grazing (76% of daily production) occurring at the end of the bloom. Large copepod species were not major grazers of the spring bloom. Greater utilization of spring bloom production by copepods in the western Irish Sea compared to regions of the North Sea is attributed to differences in population size at the time of the bloom.      

Spring bloom dynamics in Kongsfjorden,Svalbard: nutrients,phytoplankton, protozoans and primary production

The marine ecosystem in Kongsfjorden (79°N), a glacial fjord in Svalbard, is to a large extent well known with regard to hydrography, mesozooplankton and higher trophic levels. Research on primary production and lower trophic levels is still scare and especially investigations from winter and spring periods. The spring bloom dynamics in Kongsfjorden were investigated in 2002. The development in nutrient conditions, phytoplankton, protozoans and primary production were followed from 15 April until 22 May. The winter/spring in 2002 was categorized as a cold year with sea ice cover and water masses dominated by local winter-cooled water. The spring bloom started around 18 April and lasted until the middle of May. The bloom probably peaked in late April, but break-up of sea ice made it impossible to sample frequently in this period. Diatoms dominated the phytoplankton assemblage. We estimated the total primary production during the spring bloom in 2002 to range 27–35 g C m⁻². There was a mismatch situation between the mesozooplankton and the phytoplankton spring bloom in 2002.     

Spatio-temporal variability of the winter phytoplankton distribution across the Catalan and North Balearic fronts (NW Mediterranean)

The qualitative and quantitative composition of the phytoplankton assemblages of the Catalano-Balearic Sea (NW Mediterranean) was examined in relationship with frontal boundaries and short-term meteorological forcing originated by an intense wind storm. Data were obtained during February 1990, before and after strong NW winds, from several transects across the Catalan (continental side) and North Balearic (Balearic Islands side) shelf/slope fronts. High chlorophyll concentrations and diatom dominance inshore of the Catalan Front were typical of the winter-spring phytoplankton maximum. However, offshore of the Catalan Front, diatoms were scarce and the phytoplankton assemblage was dominated by coccolithophorids. In spite of weak density stratification, a deep chlorophyll maximum (DCM) was well developed, before the storm, offshore and appeared to cause a pulse of new production, as suggested by increased oxygen concentrations in the vicinity of the DCM. The results presented here indicate the existence of a persistent association between the distribution of distinct phytoplankton assemblages and hydrographic characteristics of the studied area. Our results also show that, at least during an important part of the winter-spring bloom period of the Catalano-Balearic Sea, high phytoplankton biomass offshore of the Catalan Front is not associated with the diatom dominance typically found in coastal waters and other offshore areas of the NW Mediterranean. This finding has biogeochemical implications in relationship with vertical export fluxes of particulate matter.      

Spatial and seasonal changes of dissolved and particulate organic C in the North Sea

Sampling of the central region of the North Sea was carried out to study the spatial and seasonal changes of dissolved and particulate organic C (DOC and POC, respectively). The surface waters were collected during four cruises over a year (Autumn 2004–Summer 2005). DOC and POC concentrations were measured using high temperature catalytic oxidation methods. The surface water concentrations of DOC and POC were spatially and temporally variable. There were significantly different concentrations of DOC and POC between the inshore and offshore waters in winter and summer only, with no clear trend in autumn and spring. Highest mean concentrations of DOC were measured in spring with lower and similar mean concentrations for other seasons. POC showed a clear seasonal cycle throughout the year with highest surface mean concentrations found in autumn and spring, but lowest in winter and summer. The DOC distributions during autumn and spring were strongly correlated with chlorophyll suggesting extracellular release from phytoplankton was an important DOC source during these two seasons. The lower concentrations of DOC in summer were probably due to the heterotrophic uptake of labile DOC. The seasonal changes in the C:N molar ratios of surface DOM (dissolved organic matter) resulted in higher mean C:N molar ratios in spring and lower ratios in winter. These high ratios may indicate nutrient limitation of heterotrophic uptake immediately after the spring bloom. There is limited data available for DOC cycling in these productive shelf seas and these results show that DOC is a major component of the C cycle with partial decoupling of the DOC and DON cycling in the central North Sea during the spring bloom. Handling editor: Luigi Naselli-Flores     

Effects of different N- and P-loading on primary and secondary production in an experimental marine ecosystem

The effects of nutrient loading on phytoplankton, zooplankton and macrozoobenthos in experimental ecosystems was studied in a 7-month experiment. The mesocosms were designed to mimic the major physical characteristics (irradiance, temperature, mixing) of the Dutch coastal zone in the river Rhine plume. Three different nutrient loading scenarios were used, representing present and future conditions. The level of the spring phytoplankton bloom was determined by phosphorus loading, whereas during summer the nitrogen loading determined phytoplankton biomass. The differences in nutrient loading did not result in shifts in phytoplankton species composition. With exception of the early phase of the spring bloom, diatoms dominated phytoplankton biomass in all nutrient treatments. This was ascribed to microzooplankton grazing on smaller algal species. Microzooplankton biomass showed a positive correlation with primary production, and also significant differences between nutrient treatments. Copepod development was limited, probably due to competition with microzooplankton and predation by benthic fauna. Macrobenthos biomass correlated with primary production, and was lower in the lowest nutrient treatment.     

微微型藻华爆发海域硅酸盐与叶绿素a分布特征研究

以南戴河近岸海域为研究对象,分析了该海域2009年5~10月份硅酸盐和叶绿素a的时空分布特征,并初步探讨了它们与微微型藻华的关系。结果表明:整个调查期间该海域硅酸盐平均含量为0.67±0.31 mg·L^-1,叶绿素a平均含量为2.06±1.16 ug·L^-1;其平面分布呈现近岸高、远岸低的特点;硅酸盐平均含量9月最高、5月最低;叶绿素a平均含量在8~9月高、5~6月低;微微藻赤潮期间海域硅酸盐和叶绿素a平均含量明显高于未发生微微藻赤潮的2008年同期。叶绿素a与硅酸盐浓度呈现出正相关关系,在微微藻赤潮爆发期间尤为显著,可能是因为微微型浮游植物大量生长抑制了硅藻等消耗硅酸盐藻类的正常生长繁殖所致。     

Effects of zooplankton on nutrient availability and seston C : N : P stoichiometry in inshore waters of Lake Biwa, Japan

Forty-eight-hour experimental manipulations of zooplankton biomass were performed to examine the potential effects of zooplankton on nutrient availability and phytoplankton biomass (as measured by seston concentration) and C : N : P stoichiometry in eutrophic nearshore waters of Lake Biwa, Japan. Increasing zooplankton, both mixed-species communities and Daphnia alone, consistently reduced seston concentration, indicating that nearshore phytoplankton were generally edible. The zooplankton clearance rates of inshore phytoplankton were similar to rates measured previously for offshore phytoplankton. Increased zooplankton biomass led to increased concentrations of nutrients (NH₄-N, soluble reactive phosphorus [SRP]). Net release rates were higher than those found in previous measurements made offshore, reflecting the nutrient-rich nature of inshore seston. Zooplankton nutrient recycling consistently decreased TIN : SRP ratios (TIN = NH₄ + NO₃ + NO₂). This effect probably resulted from the low N : P ratios of nearshore seston, which were lower than those commonly found in crustacean zooplankton and thus resulted in low retention efficiency of P (relative to N) by the zooplankton. Thus, zooplankton grazing inshore may ameliorate algal blooms due to direct consumption but tends to create nutrient supply conditions with low N : P, potentially favoring cyanobacteria. In comparison with previous findings for offshore, it appears that potential zooplankton effects on phytoplankton and nutrient dynamics differ qualitatively in inshore and offshore regions of Lake Biwa. Received: September 4, 2000 / Accepted: January 23, 2001     

Across-Shelf Transport of Bivalve Larvae: Can the Interface between a Coastal Current and Inshore Waters Act as an Ecological Barrier to Larval Dispersal?

Using an integrated physical and biological approach, we examined across-shelf advection and exchange and the associated transport of bivalve larvae in the presence of a strong coastal current separated from the coast by a stratified inshore environment. We tested the hypothesis that the interface of the coastal current and inshore waters can act as an ecological barrier to across-shelf transport of larvae but can be overcome by wind- or tidally-induced transport. Our study region in the Gulf of Maine encompasses a coastal current that diverges from the coast as it moves downshelf. The region inshore of this current is home to several species that exhibit limited recruitment in spite of extensive upshelf larval sources. Analysis of surface water temperatures and wind velocities revealed episodic decreases in temperature along the coast correlated with alongshelf (but not upwelling) winds, indicating wind-forced onshore movement of the cold coastal current. Such wind-driven onshore migrations are more common along the northern portion of the study region where the coastal current is near the coast, tidal currents are strong, and wind directions are more conducive to onshore migration, but rarer further south where the interface between inshore waters and the coastal current is further offshore and suitable wind events are less common. The distribution of bivalve larvae was consistent with the physical measurements. There was little across-shelf variation in larval abundance where the current abuts the coast, indicating strong across-shelf exchange of larvae, but strong across-shelf variation in larval density where the stratified inshore waters separate the current from the coast, indicating weak across-shelf transport of larvae. Our results suggest that the interface between the coastal current and inshore waters may constitute a major ecological barrier to larval dispersal in the southern part of the region that may only be overcome by rare, strong wind-forced events.     

The nearshore shunt and the decline of the phytoplankton spring bloom in the Laurentian Great Lakes: insights from a three-dimensional lake model

Dreissenid mussels have been hypothesized to cause selective decreases of phytoplankton in nearshore areas (nearshore shunt hypothesis) as well as the near-complete loss of the offshore phytoplankton spring bloom in some Laurentian Great Lakes. To evaluate whether mussels can reasonably be expected to mediate such changes, we extended the three-dimensional hydrodynamic-ecological model (ELCOM-CAEDYM) to include mussels as a state variable and applied it to Lake Erie (USA-Canada). Mussel-mediated decreases in mean phytoplankton biomass were highly sensitive to the assigned mussel population size in each basin. In the relatively deep east basin, mussels were predicted to decrease phytoplankton in both nearshore and offshore zones, even during periods of thermal stratification but especially during the spring phytoplankton maximum. Spatially, impacts were associated with mussel distributions but could be strong even in areas without high mussel biomass, consistent with advection from areas of higher mussel biomass. The results supported the nearshore shunt hypothesis that mussel impacts on phytoplankton should be greater in nearshore than offshore waters and also supported suggestions about the emerging importance of deep water offshore mussels. The results of this study provide an important insight into ecological role of mussels in lowering plankton productivity in some world’s largest lakes.     

Climate change and the phytoplankton spring bloom: warming and overwintering zooplankton have similar effects on phytoplankton

Indoor mesocosms were used to study the combined effect of warming and of different densities of overwintering mesozooplankton (mainly copepods) on the spring development of phytoplankton in shallow, coastal waters. Similar to previous studies, warming accelerated the spring phytoplankton peak by ca. 1 day °C^?1 whereas zooplankton did not significantly influence timing. Phytoplankton biomass during the experimental period decreased with warming and with higher densities of overwintering zooplankton. Similarly, average cell size and average effective particle size (here: colony size) decreased both with zooplankton density and warming. A decrease in phytoplankton particle size is generally considered at typical footprint of copepod grazing. We conclude that warming induced changes in the magnitude and structure of the phytoplankton spring bloom cannot be understood without considering grazing by overwintering zooplankton.     

A large and prolonged bloom of Karenia mikimotoi in Scottish waters in 2006

A prolonged bloom of Karenia mikimotoi was observed during 2006 in Scottish waters. This bloom is thought to be unique in the region in terms of its large spatial extent. From its first detection in the west of the country, the bloom moved clockwise around the coast eventually reaching the east coast and the Shetland Isles to the north. The bloom resulted in extensive mortalities of benthic organisms including annelids and molluscs and some species of fish. Farmed fish mortalities were absent but gill damage was reported. The availability of satellite remote sensing, phytoplankton counts from multiple sites, meteorological data and some water chemistry, as well as information on the physical characteristics of the sampling sites, provided an extensive spatial and temporal data set. Analysis of remotely sensed chlorophyll-a data from Aqua-MODIS indicated that this parameter is a useful early warning indicator of K. mikimotoi in shelf waters off the Scottish west coast, and suggested that the bloom developed in this region prior to its advection to coastal waters. An earth observation (EO) based harmful bloom classifier for K. mikimotoi recognised areas of elevated K. mikimotoi cell density but generated false positives in areas of high reflectance. Data were also used to evaluate, in Scottish waters, various hypotheses that exist to explain the formation of K. mikimotoi blooms including phototaxis, nutrient availability, cell transport and elevated water temperature. Specifically, we sought to evaluate if routinely collected environmental data (water temperature, insolation, wind strength and direction, and sea-loch aspect) could be used as a predictor of bloom magnitude near aquaculture facility locations, which typically lie within fjordic sea lochs. Path analysis was used to derive intuitive models linking environmental drivers to bloom magnitude. Once the effects of latitude such as northward water cooling were taken into account, only rainfall was a significant predictor of bloom magnitude at the sampling sites. Therefore, while the offshore development and progression of a bloom may be predicted from satellite information, it is likely that local hydrodynamic influences are crucial in determining its magnitude at coastal aquaculture sites.     

Comparison of inshore and offshore fish larval assemblages within the wider Yangtze River estuary (China)

It is hypothesized that fish larvae undertake vertical movements in estuarine waters. To test this hypothesis, a sampling period was selected during times when many spring‐spawning fish reach maximum abundance in coastal waters so that spring and summer variations in larval fish assemblages could be determined in the Yangtze River estuary. Six oceanographic surveys were conducted across the salinity gradient of an inshore (freshwater) and offshore area (brackish intersection) during spring (May) and summer (August) between 2010 and 2012. The fish larval community was dominated by species of Engraulidae, Gobiidae, Champsodontidae and Mugilidae. The pre‐flexion and flexion larval stages of euryhaline marine species, which are dependent on estuaries as nursery areas, were common. The brackish and marine larval assemblage was the most abundant with taxa such as Coilia mystus and Engraulis japonicus accounting for more than 57.3% of the total catch. Spatial differences in the taxonomic composition of larval fish assemblages were evident between the inshore and offshore areas. Additionally, the Yangtze River runoff regulatory functions as affected by the Three Gorges Reservoir operational mode (hydrological alternating operations) showed weak influences on fish and habitat environments. Low salinities from high freshwater inflow limited bay anchovy production in the inshore area.     

A seasonal study of the distributions of surface state variables in Liverpool Bay. II. Nutrients

Distributions of dissolved inorganic nitrate, phosphate, and silicate observed in the surface waters of Liverpool Bay during a seasonal cycle of cruises in 1975 are presented. Winter distributions were influenced by the proportion of waters from various sources, their chemistry, and the mode of circulation, all of which were temporally variable. Non-winter distributions were additionally influenced by in situ biological activity. Principal component analysis provided a preliminary means of separating and assessing the relative importance of these factors and depicting the centres of biological activity during the non-winter cruises. Different circulation modes in January and December produced distributions with little visual similarity, and patchy nutrient distributions accompanied the spring and autumn phytoplankton blooms. Nitrogen-rich industrial and domestic effluents modified the nutrient availability ratios in the receiving waters so that silicon limited diatom growth over large areas of the Bay in spring. Nitrogen became ‘bio-limiting’ in summer and autumn. The direction of the surface residual circulation before the spring bloom is proposed as the major influence governing subsequent nutrient distributions and phytoplankton populations.     

RELATING PHYTOPLANKTON BIOMASS AND PRODUCTION TO EPISODIC PHYSICAL FORCING IN SOUTHEASTERN LAKE MICHIGAN

Spatial and temporal dynamics in phytoplankton reflect of the combined effects of the physical and chemical environments and associated biological responses. Although alterations in phytoplankton are well-documented for a variety of lentic waters, the exact linkages between environmental forcing and phytoplankton assemblages remain poorly understood (particularly for coastal systems). A recurrent sediment resuspension event occurs every late winter/early spring in southeastern Lake Michigan, often extending greater than ten km in width and 300 km in length. Inherently, such a large-scale and dramatic physical process would be thought to dramatically influence phytoplankton assemblages; however, linkages between the turbidity plume and phytoplankton assemblages have been postulated, but never verified. As such, the episodic nature of the plume provided an opportunity to examine the effects of a short-term physical forcing event on coastal phytoplankton in relation to more persistent, seasonal meteorological forcing. Lake phytoplankton assemblages within and outside of the RCP were examined during the spring isothermal period from 1998 to 2000. Here, we describe results from the 1998 and 1999 field seasons characterizing the distribution of phytoplankton biomass and composition within and adjacent to the RCP and their relationship to particulate and dissolved constituents. In addition, the spatial and temporal patterns in production and photosynthetic characteristics of the phytoplankton community are examined.     

Ingestion rates of phytoplankton by copepod size fractions on a bloom associated with an off-shelf front off NW Spain

Herbivory by copepods was studied from the coast towards theocean, during a bloom in May 1994 off NW Spain.Ingestion rateswere estimated by the gut chlorophyll content method in threesize fractions. The chlorophyll content displayed significantdaily cycles. Three different water bodies were described: coastal,shelf break and oceanic; the latter two zones separated by athermo haline frontal structure. Marked differences in planktonspecies composition, vertical distribution and biological rateswere found between zones. Theiiighest phytoplankton biomass,dominated by chain- forming diatoms, occurred in the oceaniczone associated with low primary production rates. Copepod feedinghad a low effect on oceanic phytoplankton; up to 0.2% of carbonstock and<3% of carbon production was consumed daily. Incontrast, medium-sized and large copepods removed 3% of carbonstock and 12% of primary production daily near the coast, wherephytoplankton were dominated by small flagellates in activegrowth. The highest variability in both plankton compositionand ingestionrates was found in the shelf-break zone, probablydue to displacements of the front. Copepods exerted a moderatepredation pressure on phytoplankton in coastal waters. Meanwhile,the impact of copepods on the offshore bloom was negligibleand the fate of the accumulated particulate carbon would bemostly determined by sedimentationand water dynamics.     

Seasonal variations in abundance, development and vertical distribution of Calanus finmarchicus, C. hyperboreus and C. glacialis in the East Icelandic Current

The seasonal variation in abundance and development of Calanusfinmarchicus, Calanus hyperboreus and Calanus glacialis in relationto hydrography and chlorophyll (Chl) a was studied in the Arcticwaters of the East Icelandic Current to the north-east of Icelandfrom March 1995 to February 1996. The sampling was carried outat approximately monthly intervals on a transect of five stationsextending from 67°00'N, 13°55'W to 68°00'N, 12°40'W.In April, May and June, vertical distribution was also investigated.Spring warming of the surface waters began in May, with maximumtemperatures recorded in August (~5°C, mean for uppermost50 m). Below 75 m, temperature remained at <0°C throughoutthe year. The spring bloom of phytoplankton started in earlyMay and the highest Chl a concentrations were measured duringlate May to early June (~1 mg Chl a m^-3). Calanus finmarchicusdominated in terms of numbers (~75%), while C. hyperboreus dominatedbiomass (~76%). Calanus glacialis occurred only in low numbers(~1%) and was only a small portion of biomass (~0.7%). The abundanceof all species was low during the winter and peaked once duringsummer: C. finmarchicus in July (~16 000 ind. m^-2) and C. glacialisand C. hyperboreus in June (~370 and ~7700 ind. m^-2, respectively).The biomass of C. finmarchicus had two maxima, in April (~1.9g m^-2) and July (~1.5 g m^-2), while C. hyperboreus peaked inJune (~12.3 g m^-2). Calanus finmarchicus was estimated to spawnin early May at about the start of the spring bloom, while C.hyperboreus spawned prior to the spring bloom, in late Februaryto early March. On the basis of copepod stage distribution,C. finmarchicus was considered to have a 1-year life cycle andC. hyperboreus at least a 2-year life cycle.     

Productivity, photosynthetic pigments and hydrology in the coastal front of the Eastern English Channel

In the French area of the Eastern English Channel, the coastalwaters 3 or 4 miles wide are separated from offshore watersby a coastal front. The phytoplanktonic biomass (B) and production(P) are higher in coastal waters, and an increase of biomasshas been observed at the interface. This frontal biomass, lessproductive, resulted from hydrodynamic accumulation, not dueto local growth. The productivity (P/B) deficit (50%) spreadsover the external area of the front, probably because the biomassis partly constituted of exported inactive coastal phytoplankton.The productivity is higher again in offshore waters. The variationof the pigment pool indicates a stress on phytoplankton in thefront and this ergocline was not productive at the time of thetwo cruises.