Phytoplanktonic nutrient utilisation and nutrient signature in the Southern Ocean

The separation in Southern Ocean provinces of silicate excess at nitrate exhaustion and of nitrate excess at silicate exhaustion was already introduced by Kamykowski and Zentara (Kamykowski, D., Zentara, S.J., 1985. Nitrate and silicic acid in the world ocean: patterns and processes. Mar. Ecol. Prog. Ser. 26, 47–59; and Kamykowski, D., Zentara, S.J., 1989. Circumpolar plant nutrient covariation in the Southern Ocean: patterns and processes. Mar. Ecol. Prog. Ser. 58, 101–111) and our investigations of the silicate to nitrate uptake ratios confirm the earlier distinction. Oligotrophic antarctic waters mainly exhibit proportionally higher silicate removal what induces a potential for nitrate excess. The nitrogen uptake regime of such areas is characterised by low absolute as well as specific nitrate uptake rates throughout. Maximal values did not exceed 0.15 μM d⁻¹ and 0.005 h⁻¹, respectively. Corresponding f-ratios ranged from 0.39 to 0.86. This scenario contrasts strikingly to the more fertile ice edge areas. They showed a drastic but short vernal increase in nitrate uptake. Absolute uptake rates reached a maximum value of 2.18 μM d⁻¹ whereas the maximal specific uptake rate was 0.063 h⁻¹. In addition to an optimal physical environment for bloom development, accumulation of ammonium stimulated nitrate uptake in a direct or indirect way. Since ammonium build-up in surface waters traces enhanced remineralisation, release of other essential compounds during degradation of organic matter might have been the main trigger. This peak nitrate utilisation during early spring led to the observed potential for silicate excess. With increasing seasonal maturity the nitrate uptake became inhibited by the presence of enhanced ammonium availability (up to 8% of the inorganic nitrogen pool), however, and after a short period of intensive nitrate consumption the uptake rates drop to very low levels, which are comparable to the ones observed in the area of nitrate excess at silicate exhaustion.     

Patterns and controls of chlorophyll-a and primary productivity cycles in the Southern California Bight

Factors that influence the magnitude and the depth of the chlorophyll maximum layer in the ocean off Southern California are explored using observations from the long-term California Cooperative Oceanic Fisheries Investigations (CalCOFI) program. The data record is sufficiently long to reveal patterns not always evident from single stations or single cruises. Processes such as coastal wind-driven upwelling, geostrophic circulation, and annual physical and chemical cycles are illustrated to demonstrate their effect on euphotic zone nutrient availability, and subsequent phytoplankton biomass and primary productivity. In this area, where the influence of wind-driven upwelling is spatially restricted and advected waters are generally nutrient depleted, geostrophically induced upwelling and winter convection become important in determining spatial and temporal patterns of phytoplankton.     

High-resolution surveys of the biogeochemistry of the New England shelfbreak front during Summer, 2002

We present the observations from a pair of field experiments at the New England shelfbreak front in June and August of 2002, each consisting of 14 cross-frontal surveys using the Lamont Pumping SeaSoar. Measurements of the front's physical, chemical, and bio-optical characteristics were made at high spatial and temporal resolution. The front, based on water-column hydrographic distributions, was found within a few km of the 200 m isobath during both cruises. We present here composite sections, based on averages of individual sections shifted in space to a common frontal location, of the cross-frontal distributions of these properties as a measure of the mean state of the front in both June and August. The observations show the familiar temperature, salinity, and density distributions of the summertime front, dominated by surface thermal heating. Nutrient and bio-optical distributions show the combined effects of water-mass exchange and biological processes. T, S, silicate, and phosphate distributions are suggestive of cross-frontal exchange of slope- and shelf waters, although transport mechanisms and pathways are not apparent. These properties, along with nitrate and optical measures of the suspended particle distributions, show vertical displacements of isopleths as the front is approached: property contours slope upwards toward the front from the shoreward side, and downwards toward the front from the seaward side. Again, actual water-movement pathways are not constrained by these suggestive patterns. Bio-optical distributions show elevated indicators of photosynthetic efficiency both seaward and shoreward of the front, but the front itself is a minimum in biomass. Accumulation of photosynthetic biomass appears to be controlled primarily by nitrate scarcity in waters within and above the pycnocline. At the base of the pycnocline, light limitation appears to be the controlling factor, although the base of the euphotic zone is deeper than the biomass maxima and the base of the pycnocline. Mechanisms explaining this phenomenon are unclear, but tenuous evidence suggests low stratification at the depth of the 1% light level may not allow phytoplankton to optimize for the low-light, high-nutrient conditions at depth. Cross-frontal differences in nutrient and bio-optical parameters, particularly in August, suggest distinct phytoplankton assemblages, and the presence of calcite-forming or nitrogen fixing groups to the community structure, especially in very shallow waters across the front and in pycnocline waters seaward of the front.     

A study of the response of chlorophyll-a biomass to a winter upwelling event off Western Iberia using SeaWiFS and in situ data

Observations of a winter upwelling event off Western Iberia shelf/slope in the area of influence of the Western Iberia Buoyant Plume (WIBP) were conducted in February 2000. Spatial patterns and time evolution of the chlorophyll-a (chl-a) biomass are analysed, based on in situ and satellite data. SeaWiFS-derived chl-a concentration L2 products were used to track the chlorophyll front and estimate its westward migration velocity (maximum up to 29 km day⁻¹), as well as to characterize the frontal system and its evolution. A method associating the type of spectral signature of a pixel to the fraction of chlorophyll probed by SeaWiFS enabled the estimation of the chl-a biomass within error intervals. High chlorophyll concentrations (for wintertime) were observed over the shelf and slope, up to large distances to the coast. Due to the WIBP, a shallow Ekman layer developed, being nearly coincident with the stratified upper meters. The transport comprised westward advection and stretching of the plume, with little entrainment with the offshore deep mixed layer waters. The relative enlargement of the total area of the Inside-Front Zone (IFZ) during the upwelling event was roughly accompanied by the maintenance of the average biomass per unit of area, considering the water column up to depths of interest. This suggests that there was a net increase of chl-a biomass inside the water column associated with the IFZ, roughly proportional to the increase in the IFZ area. Retention of phytoplankton in the shallow stratified nutrient-rich waters of the WIBP was a key factor for this increase in chl-a biomass.     

Sequential data assimilation in an upwelling influenced estuary

We have evaluated the impact of assimilating chlorophyll, nitrate, phosphate, silicate and ammonium into a coupled 1D hydrodynamic ecosystem model (GOTM-ERSEM) in an upwelling influenced estuary. The assimilation method chosen is the Ensemble Kalman Filter (EnKF), which has been demonstrated to improve field estimates of key variables (chlorophyll, nutrients) for bulk algal bloom prediction. The 1D model has been set up for a central station inside the Ría de Vigo (Spain). Data from bi-weekly surveys are used to constrain the model. Temperature and salinity profiles are used to ensure the correct representation of the water structure through a relaxation scheme. Chlorophyll extracts and nutrients at three depths are assimilated sequentially during 1 year simulation (1991). The assimilation period includes episodes of active upwelling and downwelling. All five assimilated variables are successfully constrained and represent a large improvement on the reference simulation (without assimilation). Small divergences can be related to poorly resolved physical processes in the model. The assimilation was further evaluated by comparing observed biomass partitioning with model results. Diatoms accounted for the largest biomass update and the largest improvement in terms of percentage of variance explained (R²). This is particularly significant as they represent the 46% of the yearly integrated observed biomass of the planktonic autotrophs. Nonetheless the R² value was low for all phytoplankton groups. Bacteria and nanoflagellates showed an improvement with respect to their yearly Root Mean Square (RMS), while the other functional groups worsen or remained unaffected. Chlorophyll assimilation was responsible for most of the impact on the phytoplankton biomass with small contributions from the silicate. It had minor impact on the updates of nutrients which in turn corrected the state variables related to the detrital pool. In this current setting, combined assimilation of chlorophyll and nutrients is not sufficient to produce a skillful simulation of the phytoplankton succession.     

Dynamics of inorganic nutrient species in the Bohai seawaters

Within the frame of a Sino-German Joint Research Program, two cruises of “R/V Dong Fang Hong 2” were carried out in September–October 1998 and April–May 1999, respectively, to understand the dynamics of nutrients in the Bohai. Nutrient species (NO₃⁻, NO₂⁻, NH₄⁺, PO₄³⁻ and SiO₃²⁻) are determined colorimetrically on board for five anchor and 30 grid stations. In situ incubation experiments are performed to determine planktonic nutrient uptake and benthic exchange flux. Nutrient concentrations display short-term variability and seasonal change in the Bohai, with higher levels in shallow coastal waters than in the Central Bohai. The influence of riverine discharge on nutrient levels can be seen from salinity isopleths, nutrient distribution and species ratios. Near-bottom (nb) waters have similar nutrient concentrations as to the surface waters in the Central Bohai, whereas stratification takes place in the Bohai Strait and North Yellow Sea. In situ incubation experiments provide evidence that the uptake ratio (i.e. N, P) by phytoplankton is proportional to the ratios among nutrient species in ambient waters. Based on the data of this study and previously publications, a preliminary estimate of nutrient budgets via riverine input and atmospheric deposition is established. The results indicate that atmospheric deposition gains importance over rivers in delivering nutrients into the Bohai and sustain the new production, following recent decrease in riverine inflow caused by drought periods in North China and damming practices. A historical review of nutrient data indicates that concentrations of nitrogen increase and phosphorus and silica decrease in the Central Bohai over last 40 years. This potentially has an important influence on the health of ecosystem in Bohai (e.g. food web and community structure), though further study is needed to examine the scenario in more detail.     

Why is the southwest the most productive region of the East Sea/Sea of Japan?

The East Sea/Sea of Japan is a moderately productive sea that supports a wealth of living marine resources. Of the East Sea subregions, the southwest has the highest productivity. Various authors have proposed coastal upwelling, the Tsushima Current, the Changjiang Dilute Water, eddies, or discharge from the Nagdong River as potential sources of additional nutrients. In this paper, we propose, using satellite data from 1998 to 2006, that the biological productivity of the southwestern region is enhanced mainly by wind-driven upwelling along the Korean coast. Firstly, the climatology of seasonal patterns suggests that the enhanced chlorophyll a along the Korean coast is of local origin. Secondly, coastal upwelling is frequent in all seasons except winter. For example, along the coast of the Ulgi region, enhanced chlorophyll a due to coastal upwelling was observed for 25–92% of the time between Jun and Sep in the period 1998–2006. Thirdly, the advection of upwelled water through various pathways to the deeper basin was observed. Fourthly, there appeared to be a strong correlation between the interannual chlorophyll a variations of the coastal upwelling regions and the Ulleung Basin. The chlorophyll a patterns of both regions were closely related to the wind pattern in the upwelling regions, but not to that in the Ulleung Basin. Finally, changes in advection pathways also appeared to affect the productivity of the Ulleung Basin. Since 2004, there has been a shift in the pathways of upwelled water, and consequent increases in chlorophyll a in the Ulleung Basin were observed. This last observation requires further investigation.     

Cross-shelf circulation and phytoplankton distribution at the summertime New England shelfbreak front

We investigate aspects of the secondary (cross-shelf) circulation at the Middle Atlantic Bight shelfbreak front using high-resolution data collected on the New England Shelf in August 2002. The alongshore shelfbreak jet coincides with the front at the seaward edge of the cold pool (remnant winter shelf water) and there is a suggestion of a cross-stream convergence centered at the jet core. Despite indications of convergence we found no evidence of a surface subduction on the seaward side of the front. At depth 70 m near the shelfbreak there was a patch of chlorophyll, located within a local temperature–salinity maximum which, though significantly below the euphotic zone, appeared to be photo-acclimated and viable. The chlorophyll feature could be the result of a local subduction by a larger scale eddy circulation seaward of the front.Dye tracer experiments directly observed the convergence at the foot of the shelfbreak front and subsequent upwelling of bottom boundary layer water along the shoreward side of the shelfbreak front. But, we found no evidence that this upwelling influenced productivity at the front. Further, since there was no cross-shelf maximum in subsurface chlorophyll at the front, we conclude that this productivity is in general, sustained by a broadly distributed local vertical nutrient flux from an underlying nutrient reservoir.     

Nutrient and organic matter patterns across the Mackenzie River, estuary and shelf during the seasonal recession of sea-ice

Suspended material, nutrients and organic matter in Mackenzie River water were tracked along a 300 km transect from Inuvik (Northwest Territories, Canada), across the estuarine salinity gradient in Kugmallit Bay, to offshore marine stations on the adjacent Mackenzie Shelf. All particulates measured (SPM, POC, PN, PP) declined by 87–95% across the salinity gradient and levels were generally below conservative mixing. Organic carbon content of suspended material decreased from 3.1% in the river to 1.7% in shelf surface waters while particulate C:N concurrently decreased from 17.1 to 8.6. Nitrate and silicate concentrations declined by more than 90% across the salinity gradient, with nitrate concentrations often below the conservative mixing line. Phosphate concentrations increased from 0.03 μmol/L in the river to 0.27 μmol/L over shelf waters, thereby changing the inorganic nutrient regime downstream from P to N limitation. Dissolved organic carbon decreased conservatively offshore while dissolved organic N and P persisted at high levels in the Mackenzie plume relative to river water, increasing 2.7 and 25.3 times respectively. A deep chlorophyll-a maximum was observed at two offshore stations and showed increases in most nutrients, particulates and organic matter relative to the rest of the water column. During river passage through the Mackenzie estuary, particulate matter, dissolved organic carbon and inorganic nutrients showed sedimentation, dilution and biological uptake patterns common to other arctic and non-arctic estuaries. Alternatively, inorganic content of particles increased offshore and dissolved organic N and P increased substantially over the shelf, reaching concentrations among the highest reported for the Arctic Ocean. These observations are consistent with the presence of a remnant ice-constrained (‘stamukhi’) lake from the freshet period and a slow flushing river plume constrained by sea-ice in close proximity to shore. Nutrient limitation in surface shelf waters during the ARDEX cruise contributed to the striking deep chlorophyll-a maximum at 21 m where phytoplankton communities congregated at the margin of nutrient-rich deep ocean waters.     

An analysis of the characteristics of chlorophyll in the Sulu Sea

The seasonal and spatial variations of chlorophyll concentrations, Sea Surface Temperature (SST), wind fields and wind-induced Ekman pumping in the Sulu Sea are investigated using a set of new remote sensing measurements from October 1997 to December 2004. The results show the seasonality of chlorophyll, wind fields and SST and reveal the phytoplankton blooming events in the Sulu Sea basin during the northeast monsoon season. In summer, chlorophyll concentrations were relatively low (< 0.2 mg/m³) and distributed uniformly throughout the basin with a narrow belt of high chlorophyll concentrations along the coastal waters, particularly the coasts of Borneo and of the Sulu Archipelago. In winter, chlorophyll concentrations increased (> 0.2 mg/m³) throughout the entire basin, and phytoplankton bloomed southward to the central basin, while chlorophyll concentrations reached high levels (1 mg/m³) in the center of the blooms. One peak was observed during the northeast monsoon season each year. SSTs have significant negative correlations with chlorophyll concentrations; i.e., high and uniformly distributed in summer but lower with an obvious tongue of cold waters southward to the central basin in winter. The seasonal variation of chlorophyll concentrations and SST distribution were associated with the seasonally reversing monsoon. The winter phytoplankton blooming and the tongue of the cold waters were correlated to the vertical upwelling cold and nutrient-rich waters drawn by the northeast wind, with the center of the blooms and the location of cold tongues coinciding with the maximum of the wind speeds and the Ekman pumping velocities.     

Time-series analysis of remote-sensed chlorophyll and environmental factors in the Santa Monica–San Pedro Basin off Southern California

The time-series of remote-sensed surface chlorophyll concentration measured by SeaWiFS radiometer from September 1997 to December 2001 and the relevant hydrological and meteorological factors (remote-sensed sea surface temperature, atmospheric precipitation, air temperature and wind stress) in Santa Monica Bay and adjacent waters off southern California were analyzed using wavelet and cross-correlation statistical methods. All parameters exhibited evident seasonal patterns of variation. Wavelet analysis revealed salient long-term variations most evident in air temperature during El Niño 1997–1998 and in wind stress during La Niña 1998–1999. Short-period (<100 days) variations of remote-sensed chlorophyll biomass were mostly typical to spring seasons. Chlorophyll biomass was significantly correlated with air temperature and wind stress: an increase of chlorophyll biomass followed with 5–6-day time lag an increase of wind stress accompanied by a simultaneous decrease of air temperature. The mechanism of these variations was an intensification of phytoplankton growth resulting from the mixing of water column by wind stress and entrainment of nutrients into the euphotic layer.     

Primary productivity of the Palmer Long Term Ecological Research Area and the Southern Ocean

A major objective of the Palmer Long Term Ecological Research (Palmer LTER) project is to obtain a comprehensive understanding of the various components of the Antarctic marine ecosystem. Phytoplankton production plays a key role in this so-called high nutrient, low chlorophyll environment, and factors that regulate production include those that control cell growth (light, temperature, and nutrients) and those that control cell accumulation rate and hence population growth (water column stability, grazing, and sinking). Sea ice mediates several of these factors and frequently conditions the water column for a spring bloom which is characterized by a pulse of production restricted in both time and space. This study models the spatial and temporal variability of primary production within the Palmer LTER area west of the Antarctic Peninsula and discusses this production in the context of historical data for the Southern Ocean. Primary production for the Southern Ocean and the Palmer LTER area have been computed using both light-pigment production models [Smith, R.C., Bidigare, R.R., Prézelin, B.B., Baker, K.S., Brooks, J.M., 1987. Optical characterization of primary productivity across a coastal front. Mar. Biol. (96), 575–591; Bidigare, R.R., Smith, R.C., Baker, K.S., Marra, J., 1987. Oceanic primary production estimates from measurements of spectral irradiance and pigment concentrations. Global Biogeochem. Cycles (1), 171–186; Morel, A., Berthon, J.F., 1989. Surface pigments, algal biomass profiles and potential production of the euphotic layer—relationships reinvestigated in view of remote-sensing applications. Limnol. Oceanogr. (34), 1545–1562] and an ice edge production model [Nelson, D.M., Smith, W.O., 1986. Phytoplankton bloom dynamics of the western Ross Sea ice edge: II. Mesoscale cycling of nitrogen and silicon. Deep-Sea Res. (33), 1389–1412; Wilson, D.L., Smith, W.O., Nelson, D.M., 1986. Phytoplankton bloom dynamics of the Western Ross Sea ice edge: I. primary productivity and species-specific production. Deep-Sea Res., 33, 1375–1387; Smith, W.O., Nelson, D.M., 1986. Importance of ice edge phytoplankton production in the Southern Ocean. BioScience (36), 251–257]. Chlorophyll concentrations, total photosynthetically available radiation (PAR) and sea ice concentrations were derived from satellite data. These same parameters, in addition to hydrodynamic conditions, have also been determined from shipboard and Palmer Station observations during the LTER program. Model results are compared, sensitivity studies evaluated, and productivity of the Palmer LTER region is discussed in terms of its space time distribution, seasonal and interannual variability, and overall contribution to the marine ecology of the Southern Ocean.     

Variability of the western Galician upwelling system (NW Spain) during an intensively sampled annual cycle. An EOF analysis approach

The key features of the western Galician shelf hydrography and dynamics are analyzed on a solid statistical and experimental basis. The results allowed us to gather together information dispersed in previous oceanographic works of the region. Empirical orthogonal functions analysis and a canonical correlation analysis were applied to a high-resolution dataset collected from 47 surveys done on a weekly frequency from May 2001 to May 2002. The main results of these analyses are summarized bellow. Salinity, temperature and the meridional component of the residual current are correlated with the relevant local forcings (the meridional coastal wind component and the continental run-off) and with a remote forcing (the meridional temperature gradient at latitude 37°N). About 80% of the salinity and temperature total variability over the shelf, and 37% of the residual meridional current total variability are explained by two EOFs for each variable. Up to 22% of the temperature total variability and 14% of the residual meridional current total variability is devoted to the set up of cross-shore gradients of the thermohaline properties caused by the wind-induced Ekman transport. Up to 11% and 10%, respectively, is related to the variability of the meridional temperature gradient at the Western Iberian Winter Front. About 30% of the temperature total variability can be explained by the development and erosion of the seasonal thermocline and by the seasonal variability of the thermohaline properties of the central waters. This thermocline presented unexpected low salinity values due to the trapping during spring and summer of the high continental inputs from the River Miño recorded in 2001. The low salinity plumes can be traced on the Galician shelf during almost all the annual cycle; they tend to be extended throughout the entire water column under downwelling conditions and concentrate in the surface layer when upwelling favourable winds blow. Our evidences point to the meridional temperature gradient acting as an important controlling factor of the central waters thermohaline properties and in the development and decay of the Iberian Poleward Current.     

Distribution, reproduction and development of Calanus species in the Northeast water in relation to environmental conditions

The vertical and horizontal distribution of mesozooplankton biomass and its composition, together with the reproduction and development of the three dominant herbivorous copepods Calanus finmarchicus, C. glacialis and C. hyperboreus, were studied in the Northeast Water between the end of May and end of July, 1993. Biomass varied between 0.15 and 10.6 g m⁻² and was lower on the shallow banks. Highest biomass was found on the shelf slope and in the entrances to Belgica and Westwind Troughs. There was no seasonal trend during the study period. Among the zooplankton taxa, copepods were dominant, with 84% biomass of all other taxa, followed by chaetognaths with 14%. The large Calanus species made up 91% of copepod biomass. The most abundant species, C. glacialis, inhabited areas of low current speeds on Belgica and Ob Bank, C. hyperboreus dominated shelf slope and trough stations, while C. finmarchicus was most abundant in the Return Atlantic Current along the shelf slope and also eastern Belgica Trough. C. glacialis was the only Calanus species spawning during this period, but young copepodites of the other species were also abundant. Egg production of C. glacialis was at a maximum by our arrival and continued to at least mid August. According to the results from starvation experiments, its egg production was fuelled by food uptake, but was decoupled from phytoplankton chlorophyll until July, indicating ice-algae and microzooplankton as an alternative food source. Only when the polynya approached its maximum extent was a close relationship to phytoplankton established. Due to both spatial and temporal heterogeneity of the occurrence of young copepodite stages it was impossible to follow the growth of cohorts of developmental stages. Herbivorous carbon requirements estimated from egg production rates were mostly less than one third of the phytoplankton stock. From egg production and the distribution of young stages, the outer Westwing Trough seems to be the centre of biological activities. This may be related to the formation of young ice in winter in this area, which was found to carry a large mass of under-ice algae. High biomass but low production in Belgica Trough indicate this as an advective and expatriated community of C. finmarchicus and C. hyperboreus, where grazing is negligible, at least during early summer.     

Ecology of bottom ice algae: II. Dynamics, distributions and productivity

Spring blooms of bottom ice algae are a common feature of landfast congelation ice in polar regions. Because ice algae are usually associated with a substrate, their population dynamics can be followed with considerable confidence. Although ice algal dynamics are closely related to irradiance, their dynamics and distributions are influenced by other abiotic and biotic factors. Ice algal abundance varies horizontally over all scales examined. Factors such as grazing and nutrient availability may contribute to local and geographic differences. Loss terms for most sea ice assemblages are largely unquantified. Ice algal biomass is most concentrated near the ice-water interface in spring.Environmental factors affecting ice algal abundance and productivity are considered here, emphasizing recent results from several well-studied sites. Biomass accumulation, growth rates and productivity have been documented for spring blooms of bottom interstitial and sub-ice assemblages. On an areal basis biomass accumulation in bottom ice assemblages can be comparable with planktonic systems. At low ambient temperatures and irradiances average specific growth rates (≤ 0.25 d⁻¹) and production rates (≤ 1.0 mg C mg Chl⁻¹ h⁻¹) for ice algae are low. Current methods of measuring productivity are compared. Results are consistently low but variable with little systematic difference among them. At present, apparent differences in productivity between bottom ice assemblages in the Arctic and Antarctic, or among different antarctic assemblages, are so confounded by methodological and other sources of variability that no firm differences can be detected.     

Quantitative estimation of the influence of surface thermal fronts over chlorophyll concentration at the Patagonian shelf

Eighteen-year (1985–2002) mean monthly SST Pathfinder data with 9 km spatial resolution have been used to estimate surface gradients by finite differences. Then the seasonal climatological means have been calculated from the intensity of these gradients, and surface thermal fronts present in the Patagonian Continental Shelf (PCS) have been located. Moreover, 6 years (1998–2003) of SeaWiFS data with approximately 4 km spatial resolution have been used to estimate monthly composite images of surface chlorophyll concentration, after which seasonal climatological means distributions have been generated. Both seasonal distributions have been analyzed together and by combining the knowledge of oceanographic processes and phytoplankton responses to light and nutrient availability, regions where the presence of a thermal front affects photosynthetic activity have been identified. Subjective criteria have been applied to define eighteen areas where phytoplankton biomass is influenced by the presence of a thermal front. In these areas, the surface chlorophyll (spatial mean and total), its relationship with the surface chlorophyll of the whole region, and the seasonal evolution of this relationship have been calculated. All frontal areas cover less than 15% of the total surface, but they contribute with over 23% of the phytoplankton annual mean biomass. Considered as a group, during summer they show high chlorophyll values very similar to those in spring. During the cold period, when the water column is vertically mixed in practically the whole of PCS, the influence of physical fronts over the biological production is minimum. The frontal zone image remains clearly defined during summer, when approximately 85% of the area will have a determined mean chlorophyll concentration, while the other 15% has a 2.45 times larger value. While three pattern trends have been identified in the frontal areas, only two of them condition the pattern of the group, due to their horizontal extension.     

Vertical profiles of particulate organic matter and its relationship with chlorophyll-a in the upper layer of the NE Mediterranean Sea

Particulate organic matter (POM), nutrients, chlorophyll-a (CHL) and primary production measurements were performed in the upper layer of three different regions (cyclonic, anticyclonic and frontal+peripherial) of the NE Mediterranean Sea in 1991–1994. Depth profiles of bulk POM exhibited a subsurface maximum, coinciding with the deep chlorophyll maximum (DCM) established near the base of the euphotic zone of the Rhodes cyclone and its periphery, where the nutricline was situated just below the euphotic zone for most of the year. Moreover, the POM peaks were broader and situated at shallower depths in late winter–early spring as compared to its position in the summer–autumn period. Under prolonged winter conditions, as experienced in March 1992, the characteristic POM feature disappeared in the center of the Rhodes cyclone, where the upper layer was entirely occupied by nutrient-rich Levantine deep water. Deep convective processes in the cyclonic gyre led to the formation of vertically uniform POM profiles with low concentrations of particulate organic carbon (POC) (2.1 μM), nitrogen (0.21 μM), total particulate phosphorus (PP) (0.02 μM) and chlorophyll-a (0.5 μg/L) in the euphotic zone. Though the Levantine deep waters ascended up to the surface layer with the nitrate/phosphate molar ratios (28–29) in March 1992, the N/P molar ratio of bulk POM in the upper layer was low as 10–12, indicating luxury consumption of phosphate during algal production. Depth-integrated primary production in the euphotic zone ranged from 38.5 for oligotrophic autumn to 457 mg C m⁻² day⁻¹ for moderately mesotrophic cool winter conditions.     

Application of a validated primary production model (BLOOM) as a screening tool for marine, coastal and transitional waters

In order to manage aquatic systems, it is necessary to apply methods relating the environmental variables and system-state parameters with external factors that affect the system. External factors can be natural (i.e. the movement of water) or partly-anthropogenic (i.e. nutrient loads). In addition to the national authorities, who have been implementing environmental policies for several decades, the EU is presently implementing the Water Framework Directive (WFD) aimed at establishing a new set of standards for the ecological and water quality of water systems. Among these are the phytoplankton biomass and composition. Phytoplankton affects turbidity, oxygen depletion, total productivity of the system and the occurrence of (harmful) algal blooms. A range of methods is available to relate phytoplankton to the controlling environmental conditions. Among these are statistical relations for instance of the Vollenweider type as well as deterministic simulation models. At the end of the 1970s, a generic deterministic phytoplankton module called BLOOM was developed, which has since been applied to a wide range of fresh water and marine systems. Here we test the applicability of this model as a screening tool for coastal waters. We conclude that the model is able to reproduce observed chlorophyll levels adequately under a wide range of conditions. Subsequently the model is applied to demonstrate the potential impacts of reductions in nitrogen, phosphorus or both nutrients simultaneously. Depending on which factors are initially controlling, the impacts of these reductions vary considerably both between locations and during the season. While this type of application lacks explicit relations between nutrient concentrations and external loadings, it does consider a number of relevant conditions in a consistent way and requires remarkably little data and effort. It is therefore a valuable screening tool.     

Variability of the shelf water off the northeast Spanish coast

During the seasonal stratification of 1983 and 1984, intensive sampling of the shelf region of the northeast Spanish coast was carried out. This unique dataset is used to define the most salient features of the shelf water dynamics. We show that region has a strong spatial and temporal variability associated with the southward spreading of continental waters from the Rhone river, in the northern gulf of Lyons. Southward spreading begins in early spring and was particularly obvious during June 1983 when strong horizontal density gradients were observed. Spreading almost completely disappears in August when observed upper layer salinities are uniform. We also show that significant shelf/slope water exchange takes place associated with the abrupt topographic canyons found in the region. In particular we observed in June 1983 a small positive geostrophic vorticity region at the Palamós canyon while a larger anticyclonic eddy was observed over the shelf south of this canyon. The biological implications of these two features have been demonstrated. We conclude indicating that in this region where no dominant winds exist, the shelf water dynamics is dominated by the southward spreading of low salinity continental waters and by the intrusion of high salinity open ocean water through submarine canyons.     

Surface chlorophyll in the Black Sea over 1978–1986 derived from satellite and in situ data

Absolute values of chlorophyll a concentration and its spatial and seasonal variations in the Black Sea were assessed by using satellite CZCS and in situ data. Since the satellite CZCS had operated for the 1978–1986 period, CZCS data was used for assessing the past state of the Black Sea just before the onset of drastic changes observed in late 1980s. The approach used for the calculation of the absolute values of chlorophyll a concentration from CZCS data was based on the direct comparison of in situ chlorophyll a data and those of CZCS and by applying the algorithm developed for the transformation of CZCS data into chlorophyll a values. CZCS Level 2 data related with pigment concentration having a spatial resolution of 1 km at nadir were used. The daily Level 3 files were derived by binning Level 2 values into 4-km grid cells and the monthly and seasonal Level 3 files were created by averaging the daily Level 3 files over the corresponding period. In situ chlorophyll a data were obtained by spectrophotometric and fluorometric methods in 15 scientific cruises over the 1978–1986 period. Total number of ship-measured data used for the comparison with those CZCS values was 590.Chlorophyll a concentration (Chl) was derived from CZCS values (C) with regression equations Chl=kC; the coefficient of transformation k was calculated from six different data sets by taking into account distinctions between subregions and seasons. The reasons for difference in the k values have been analyzed.Statistical comparison of the chlorophyll a values measured in situ and those derived from CZCS data was based on log-transformed data and gave the following results: regression SLOPE=0.842, regression INTERCEPT=−0.081, coefficient of determination (R²)=0.806, root–mean–square ERROR=0.195. The mean monthly chlorophyll a distributions derived from CZCS data over 1978–1986 have been constructed and the mean seasonal chlorophyll a values in different regions have been calculated and analyzed. The significant difference in chlorophyll concentration between the western shelf regions and the open part of the Black Sea has been demonstrated, especially in warm season. At almost all seasons, the highest chlorophyll concentration is observed in the western interior shelf region which is under strong influence of Danube. The summer mean chlorophyll concentration in this region is 18 times higher than that in the open parts and about nine times higher than in the eastern shelf region. The greatest seasonal variations are observed in the open part of the Black Sea: chlorophyll concentration in cold season is four to six times higher than in summer and three to five times higher than in April and October. To the contrary, in the western interior shelf regions, the concentration is higher in May–October (about twice than that in November–March). Seasonal variations in the western outer shelf regions are smoothed out as compared with both the western interior shelf and the open regions.