Average Chlorophyll Concentration Research Articles

Long-term dynamics of chlorophyll concentration in the ocean surface layer (by space data)

To preserve the biosphere and to use it efficiently, it is necessary to gain a deep insight into the dynamics of the primary production process on our planet. Variability of chlorophyll concentration in the ocean is one of the most important components of this process. These investigations are, however, very labor-consuming, because of the difficulties related to the accessibility of the water surface and its large size. In this work long-term changes in chlorophyll concentration in the surface layer of the ocean have been analyzed on the basis of the CZCS data for 7.5 years from 1979 to 1986 and the SeaWiFS data from 1997 to 2004. It has been shown that the average chlorophyll concentration calculated in all investigated areas varies moderately. However, when analyzing spatially local trends, the areas have been detected that have significant rise and fall of chlorophyll concentrations. Some interesting features of the long-term dynamics of chlorophyll concentration have been found. The opposite directions of long-term trends (essential increase or decrease) cannot be explained only by large-scale hydrological phenomena in the ocean (currents, upwellings, etc.). The measured chlorophyll concentration results from the balance between production and destruction processes. Which process dominates is determined by various hydrophysical, hydrobiological, and climatic processes, leading to sharp rises or falls of the concentration. It is important to estimate the scale of the areas in which this or that process dominates. Therefore, the study addresses not only the dynamics of the mean value but also the dynamics of the areas in which the dominance of certain factors has led to a sharp fall or rise in chlorophyll concentration. Thus, the obtained results can be used to estimate long-term changes in the ocean biota.

Chlorophyll- a in the rivers of eastern England

Chlorophyll- a concentration variations are described for two major river basins in England, the Humber and the Thames and related to catchment characteristics and nutrient concentrations across a range of rural, agricultural and urban/industrial settings. For all the rivers there are strong seasonal variations, with concentrations peaking in the spring and summer time when biological activity is at its highest. However, there are large variations in the magnitude of the seasonal effects across the rivers. For the spring–summer low-flow periods, average concentrations of chlorophyll- a correlate with soluble reactive phosphorus (SRP). Chlorophyll- a is also correlated with particulate nitrogen (PN), organic carbon (POC) and suspended sediments. However, the strongest relationships are with catchment area and flow, where two straight line relationships are observed. The results indicate the importance of residence times for determining planktonic growth within the rivers. This is also indicated by the lack of chlorophyll- a response to lowering of SRP concentrations in several of the rivers in the area due to phosphorus stripping of effluents at major sewage treatment works. A key control on chlorophyll- a concentration may be the input of canal and reservoir waters during the growing period: this too relates to issues of residence times. However, there may well be a complex series of factors influencing residence time across the catchments due to features such as inhomogeneous flow within the catchments, a fractal distribution of stream channels that leads to a distribution of residence times and differences in planktonic inoculation sources. Industrial pollution on the Aire and Calder seems to have affected the relationship of chlorophyll- a with PN and POC. The results are discussed in relation to the Water Framework Directive.

Responses of estuarine and coastal marine phytoplankton to nitrogen and phosphorus enrichment

A cross‐ecosystem comparison of data obtained from 92 coastal zone ecosystems worldwide revealed a strong positive response of marine phytoplankton biomass to nutrient enrichment that is highly consistent with the general patterns reported previously in the limnological literature for freshwater lakes and reservoirs. Average concentrations of chlorophyll a in estuarine and coastal marine systems were strongly dependent on the mean concentrations of total nitrogen and total phosphorus in the water column. Moreover, as is true of freshwater ecosystems, the identity of the primary growth‐limiting nutrient for marine phytoplankton appeared to be generally predictable from watercolumn total nitrogen : total phosphorus (TN : TP) ratios. This similarity in physiological response to nutrients likely derives from the shared evolutionary histories of marine and freshwater phytoplankton.

Bridging ocean color observations of the 1980s and 2000s in search of long‐term trends

A comprehensive revision of the Coastal Zone Color Scanner (CZCS) data‐processing algorithms has been undertaken to generate a revised level 2 data set from the near‐8‐year archive (1979–1986) collected during this “proof‐of‐concept” mission. The final goal of this work is to establish a baseline for a global, multiyear, multisensor ocean color record, to be built from observations of past (i.e., CZCS), present, and future missions. To produce an internally consistent time series, the same revised algorithms also have been applied to the first 5 years of the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) ocean color observations (1998–2002). Such a database is necessary in order to determine whether or not the ocean biogeochemistry has evolved in the past years and, if so, to be able to detect near future trends. Algorithmic and calibration aspects, along with validation results presented in this paper, are tailored toward the identification of long‐term trends, which mandated this reprocessing effort. The analysis of decadal changes from the CZCS to the SeaWiFS era shows an overall increase of the world ocean average chlorophyll concentration by about 22%, mainly due to large increases in the intertropical areas, where the seasonal cycles also substantially changed over the past 2 decades. Increases in higher latitudes, where seasonal cycles did not change, contribute to a lesser extent to the general trend. In contrast, oligotrophic gyres display declining concentrations.

Scaling the trophic status of the Aegean Sea, eastern Mediterranean

Data on chl-a and primary production were recorded from the northern to the southern open waters and from inshore to offshore waters of the Aegean Sea. The data were analysed statistically and their levels were scaled and characterised on the basis of probabilistic parameters. The limits of average chlorophyll concentrations and productivity in open oligotrophic waters were less than those in the offshore mesotrophic and these were less than in inshore eutrophic waters. According to this scaling, the categories ‘open oligotrophic’ < ‘offshore mesotrophic’ < ‘inshore eutrophic’ waters were defined for chl-a as 0.5 < (0.5 – 1.0) < 1.0 mg m − 3, and for primary production as 1.5 < (1.5 – 3.0) < 3.0 mg C m − 3 h − 1. The comparison of the mean depth of the euphotic zone also displayed a characteristic gradient from oligotrophic open waters (90–129 m) to mesotrophic offshore (35 m) and eutrophic inshore (7 m) waters. Eutrophication is a major problem in most of the European regional seas including the Mediterranean and the Black Sea. The classification of the trophic status of seawater is important because it is central to ecological studies of eutrophication and its practical role as an indicator of water quality.

Overwintering growth and development of larval Euphausia superba: an interannual comparison under varying environmental conditions west of the Antarctic Peninsula

Growth, molting, and development of larval Antarctic krill were investigated near Marguerite Bay during four cruises in austral autumn and winter 2001 and 2002, as part of the US Southern Ocean GLOBEC program. Overwintering survival of larvae has been linked to annual sea-ice formation and extent, as sea-ice biota may provide food when other sources are scarce in the water column. During autumn 2001, larvae were very abundant (1–19 individuals m � 3 ), with younger stages dominant offshelf and older stages dominant on-shelf. On-shelf larvae were in better condition than offshore larvae. During autumn 2002, larvae again were abundant offshelf (0.01–110 m � 3 ), whereas all stages were scarce on-shelf. Declining diatom and radiolarian blooms were present during autumn in both years. Average chlorophyll concentrations were low (0.10 vs. 0.22m gl � 1 ) in autumn and an order of magnitude lower in

Linking habitat conditions and growth in the European anchovy (Engraulis encrasicolus)

Abstract Age structure and growth parameters for the anchovy fishery, Engraulis encrasicolus , were estimated for the first time in the Strait of Sicily. Environmental factors including sea surface temperature and chlorophyll concentrations were considered in order to explain differences in growth parameters between different areas. The sampling was carried out from May 2000 to October 2001 on board commercial fishing vessels. The age structure of the catches was dominated by the second-year-class (55–63%) and the first-year-class (30–41%), whereas the 0-year-class (1–5%) and the third-year-class (3–5%) represented a small proportion of the catches. The von Bertalanffy growth model and back-calculation analyses were applied to estimate anchovy growth parameters using the FiSAT program. There were not significant differences between males and females, pooled mean parameters were L ∞ =18.6 cm, k =0.29 per year and t 0 =−1.81 years. The estimated parameter k of the Sicilian anchovy was at the lower end of the range observed for this fish species in different areas, ranging from 0.26 (Algeria) to 2.44 (north Adriatic sea). Average chlorophyll concentrations (from 1997 to 2002), and sea surface temperatures (from 1971 to 2000) derived from satellite images for each area, explained the 88% of the variance observed in the growth of the anchovy between the areas. As fish production results basically from recruitment success and fish growth, we conclude that specifically for each species, relationships between fish growth with habitat conditions (mainly primary productivity and temperature) could be used to assess the potential fishery yields of a region, in addition to considering all factors that potentially could affect recruitment success in that region.

Prediction of ocean colour: Monte Carlo simulation applied to a virtual ecosystem based on the Lagrangian Ensemble method

The annual variation of ocean-colour signals in the North Atlantic Ocean is successfully simulated by combining the plankton ecosystem model and the optical model. The first model uses the Lagrangian Ensemble method of Woods and Barkmann to simulate the upper-ocean ecosystem, including the vertical profile of chlorophyll concentration. The second model employs the Monte Carlo technique to compute the optical environment for that virtual ecosystem with 31 wavebands in the visible spectrum (400–700 nm). Ocean-colour signals are determined from the spectrum of the relatively few photons that are scattered back up through the sea surface. This information is then substituted into various satellite ocean-colour algorithms to calculate the satellite-derived surface chlorophyll concentration. Results show that substantial differences exist among the predictions from different ocean-colour algorithms. In addition, the average chlorophyll concentration of the upper few layers is not equal to the satellite-derived surface chlorophyll concentration. Our research encourages the adoption of the Monte Carlo optical model to simulate the satellite ocean-colour signals for the purpose of evaluating the plankton ecosystem model.

Impact of the intensive shrimp farming on the water quality of the adjacent coastal creeks from Eastern China

The effects of shrimp aquaculture on the adjacent water bodies over a crop cycle period were evaluated by studying the water quality of inlet and outlet creeks located within Xuwei Salt Field, Lianyungang City of Jiangsu Province. The characterization of the water consisted of the evaluation of the variation of 10 parameters along the inlet and outlet creeks and during the period of March–September. These parameters were pH, temperature, dissolved oxygen, nutrients (ammonium, nitrite, nitrate and inorganic phosphorus), Chlorophyll a, chemical oxygen demand (COD) and total organic carbon (TOC). Average concentrations of inorganic nitrogen (1.5 mg/l), inorganic phosphorus (0.024 mg/l), Chlorophyll a (0.0025 mg/l), COD (5.77 mg/l) and TOC (6.57 mg/l) at the discharge site in the outlet creek were significantly higher than in the inlet creek (0.33, 0.012, 0.0018, 4.72 and 3.68 mg/l, respectively). There were no significant differences between the inlet and outlet creeks in pH, temperature and DO. With a water quality criteria based on local laws, a significant pollution was observed in the area, with inorganic nitrogen being the most significant pollutant, followed by COD and inorganic phosphorus.

Predicting the Frequencies of High Chlorophyll Levels in Florida Lakes from Average Chlorophyll or Nutrient Data

ABSTRACT An alternative to using average concentrations of phytoplankton chlorophyll in setting lake management goals is to use the frequency that chlorophyll concentrations exceed an established nuisance level. We analyzed 1473 lake-years of data on 438 Florida lakes to develop a series of tables that can be used to predict the frequencies that phytoplankton chlorophylls will exceed concentrations of 10, 20, 30, 40, 50, and 60 mg m−3 in Florida lakes based on the annual average concentrations of chlorophyll, total phosphorus, or total nitrogen. Different tables were created for lakes grouped by TN/TP ratios of >17, <17 but >10, and <10. These tables can be directly applied towards setting lake management goals for those Florida lakes that lack sufficient data to construct their own bloom frequency curves.

Long-term changes in plankton community structure and productivity in the North Pacific Subtropical Gyre: The domain shift hypothesis

Oceanic productivity, fishery yields and the net marine sequestration of atmospheric greenhouse gases are all controlled by the structure and function of planktonic communities. Detailed paleoceanographic studies have documented abrupt changes in these processes over timescales ranging from centuries to millennia. Most of these major shifts in oceanic productivity and biodiversity are attributable to changes in Earth's climate, manifested through large-scale ocean–atmosphere interactions. By comparison, contemporary biodiversity and plankton community dynamics are generally considered to be “static”, in part due to the lack of a suitable time frame of reference, and the absence of oceanic data to document ecosystem change over relatively short timescales (decades to centuries). Here we show that the average concentrations of chlorophyll a (chl a) and the estimated rates of primary production in the surface waters of the North Pacific Subtropical Gyre (NPSG) off Hawaii have more than doubled while the concentrations of dissolved silicate and phosphate have decreased during the past three decades. These changes are accompanied by an increase in the concentration of chl b, suggesting a shift in phytoplankton community structure. We hypothesize that these observed ecosystem trends and other related biogeochemical processes in the upper portion of the NPSG are manifestations of plankton community succession in response to climate variations. The hypothesized photosynthetic population “domain shift” toward an ecosystem dominated by prokaryotes has altered nutrient flux pathways and affected food web structure, new and export production processes, and fishery yields. Further stratification of the surface ocean resulting from global warming could lead to even more enhanced selection pressures and additional changes in biogeochemical dynamics.

Criticism of the litterbag technique for the study of aquatic plant decay: Suppression of epiphytic algal biomass.

Recent studies have suggested thiat autotrophs (cyanobacteria and eukaryotic algae) attached to emergent plant detritus are important to the decay of that detritus; however, the litterbag methodology commonly employed for study of plant decay may inhibit epiphytic autotrophs. Thus, the purpose of this study was to examine the change in epiphytic algal biomass on Typha latifolia litter in four types of litter enclosures. The enclosures were designed to provide various combinations of light intensity and invertebrate access to the surface of enclosed Typha litter. Within the enclosures, algal biomass (as chlorophyll-a) on the litter and rate of Typha decay were examined over 133 days (May-October, 1994). Over the entire study, average chlorophyll concentrations on litter incubated in enclosures constructed of fine mesh ranged from 0.41 ± 0.04 to 0.23 ± 0.04 μg cm -2 of detritus, relative to values of 0.63 ±0.07 to 0.75 ± 0.07 μg cm -2 on detritus incubated in coarse-mesh enclosures or open, floating enclosures. The results clearly indicated that autotrophic biomass on Typha litter was inhibited by litter enclosures of fine mesh; consequently, previous studies of aquatic plant decay may have missed contributions to decay processes by this important component of the detrital community.

Tintinnid ciliate diversity in the Mediterranean Sea: longitudinal patterns related to water column structure in late spring-early summer

Globally, north-south gradients in diversity are well known, and in the world ocean they have been related to north-south gradients in water column structure. Here diversity in planktonic ciliates, both taxonomic and morphological, is explored in the Mediterranean Sea, which displays west-east gradients in the water column characteristics of chlorophyll concentrations and distribu- tions. Tintinnids, which are loricate planktonic ciliates, were enumerated and identified in samples from 23 stations between 2 and 32° E, obtained from the Catalan Sea in June 1993 and from a cross- Mediterranean transect in May-June 1996. From west to east, concentrations of tintinnids showed little variability, while numbers of species and genera, as well as diversity indices (H' values), increased. These parameters were positively related to depths of the chlorophyll maximum layer and negatively correlated with chlorophyll concentrations. Corresponding to the west to east increases in diversity were increases in community averages of lorica oral diameter and overall length, again positively cor- related to deepening of chlorophyll maximum depths, and negatively related to average chlorophyll concentrations. Morphological diversity was related to species diversity, not as variability in lorica oral diameters, but as variability in average lorica length. In the Mediterranean Sea, a gradient of taxo- nomic and morphological diversity is relatable to a specific feature of the water column, the depth of the chlorophyll maximum layer. Trophic specialization as a dominant mechanism underlying diver- sity was not supported by data on lorica oral diameters.

A Strategy for Reservoir Model Forecasting Based on Historic Meteorological Conditions

ABSTRACT A strategy for the application of linked watershed and reservoir models in the analysis of water quality management proposals for a water supply reservoir is presented. This strategy is based on the use of a long-term historical record of meteorological data, so that die predicted changes in water quality may be evaluated by considering the variations in streamflow, material loading, reservoir heat transfer and mixing associated with natural variations in meteorological conditions. Model simulations for a baseline condition and for several management proposals involving point source nutrient control, nonpoint source nutrient reduction, and reservoir operations are presented. The predictions are presented as distributions of the frequency of occurrence of selected annual statistics of nutrient loading, reservoir stratification, and reservoir water quality. Simulations for Cannonsville Reservoir indicate that reductions in phosphorus loading from wastewater treatment plants in the watershed would have a greater impact on summer average chlorophyll concentrations than nonpoint phosphorus control, and that reservoir operations that result in a decrease in reservoir drawdown result in improved reservoir water quality.

Oceanographic characteristics of an impacted coastal bay: Baía de Guanabara, Rio de Janeiro, Brazil

Baía de Guanabara is a 384 km 2 eutrophic coastal bay in Brazil, impacted by the polluted discharge from the Rio de Janeiro metropolitan area. The structurally controlled bay has a central channel with a depth of 30 m and a sandy bottom near the entrance, reflecting wave and tidal forcing. In contrast, the bay-averaged water depth is 5.7 m and the bottom sediments are mostly muds as a result of the Holocene transgression and rapid fluvial sedimentation, accelerated by channelization of rivers and deforestation. An extensive sand bank is located seaward of the bay entrance and a flood-oriented sand wave system indicates sand transport into the bay. The mean freshwater discharge measures 100±59 m 3 s −1 and is greatest in the rainy austral summer in December and January. Tides are mixed mainly semidiurnal with a range of 0.7 m, and peak spring tidal currents reach 0.5 m s −1 inside the bay and 1.6 m s −1 near the bay entrance. The passage of northward propagating polar fronts results in regular strong southwesterly winds and heavy wave forcing. The bay has mean salinities from 21.0 to 34.5‰ with an average of 29.5±4.8‰. The vertical salinity stratification, Δs/s, varies from 0.06 to 0.21 and is relatively weak and inversely proportional to rms tidal currents. The residual circulation is characterized by both gravitational circulation and transverse residual tidal circulation, measuring 800 and 400 m 3 s −1 respectively. The renewal time of 50% of the bay water volume is 11.4 days. Untreated sewage runoff enters the bay from the west, resulting in locally poor water quality, where the near-bottom mean dissolved oxygen measures only 3.1 mg 1 −1 and results in anoxic bottom muds. The worst water quality is indicated by average fecal coliform of 1140 counts ml −1 and excessive ammonia and phosphate loading. The average chlorophyll concentration in this region responds to the nutrient loading and exceeds 130 μg 1 −1 although 57 μg 1 −1 is the overall mean for the bay. The atomic N:P ratio measures 14 for the bay as a whole.

CAUSES OF UGHT AVI'ENUATION IN TAMPA BAY AND CHARLOTTE HARBOR, SOUTHWESTERN FLORIDA1

ABSTRACT: Vertical attenuation of photosynthetically active radiation (PAR) in clear waters of central Florida theoretically can vary almost 50 percent during a sunny summer day as a result of changing solar elevation. We used a simple formula to partially adjust the attenuation coefficient in Tampa Bay and Charlotte Harbor for changing solar elevation of the direct beam and then used multiple regression analysis to estimate the relative contribution of different water properties or constituents to the adjusted attenuation coefficient, kadj. Color, on an average, was responsible for 18 percent of kadj, chlorophyll a for 21 percent, nonchiorophyll suspended matter for 55 percent, and seawater for the remaining 6 percent. In both estuaries, kadj increased with decreasing salinity as a result of freshwater runoff adding color, suspended matter, and nutrients. Nutrients affected attenuation by stimulating phytoplankton growth and increasing concentrations of chlorophyll a. Reduced nutrient loading to upper Tampa Bay (Hilisborough Bay) in the early to mid‐1980's appears to have decreased concentrations of chlorophyll a, increased water clarity, and increased seagrass recolonization. Assuming other attenuating substances remained unchanged, the decrease in the average concentration of chlorophyll a from 30 to 15 μg L−1 would correspond to an increase in the depth of light penetration necessary for seagrass survival (&gt;10 percent incident light) from 1.0 to 1.5 m, which, on a relatively flat sea bed (slope of 2 m/km), would increase the area potentially available for seagram recolonization by 0.25 km2/km of shoreline.

Long-Term Nutrient Loading and Biological Response in a Flood-Water Reservoir (Neue Donau) in Vienna, Austria

Nutrient input and trophic conditions in an artificial flood protection reservoir along the river Danube in Vienna are evaluated from a 10-year data base. Nutrient concentrations were largely governed by concentrations in the River Danube (total phosphorus, P-tot = 236±41 µg/l; total nitrogen, N-tot = 2.53±0.78 mg/l), because the major input was groundwater percolating from the river through the separating island to “Neue Donau”. Concentrations were reduced by about 40% during subsurface transport (time lag approx. one day), because the sediments were mainly composed of gravel (correlation coefficient r = 0.77 for P-tot of the river vs. Neue Donau). The amount of water strongly depended on the complex hydrological regime. The decreasing concentration gradients that developed downstream in the Neue Donau are analysed in detail for the year 1991, for total phosphorus, chlorophyll-a, and phytoplankton biomass. Long-term changes of these parameters are discussed for the years 1981 to 1991. Average phosphorus concentrations for the summer period (June - September) increased from &amp;gt;100 µg/l in 1981 to &amp;gt;400 µg/l in 1984. Average chlorophyll concentrations ranged from 40 to 80 µg/l during this period, peaking in the year 1985. Both variables declined drastically in the years 1986 and 1987 and more slowly decreased thereafter. Lowest values of 40 and 10 µg/l respectively were reached in 1990 which were not significantly different from both 1989 and 1991.

Nutrient cycling and productivity in a desert saline lake: observations from a dry, low-productivity year

Seasonal variability of nutrients and productivity were examined in Pyramid Lake, a hyposaline, N-deficient, terminal desert lake, during a dry period. River inflow and N-fixation during 1990 were minimal allowing internal nutrient cycling to be more closely studied. Nutrient cycling was strongly affected by seasonal thermal stratification that was typical for a warm monomictic lake. Concentrations of nitrate, phosphate, and silicate in surface waters were highest during winter mixing and decreased rapidly in the spring due to a diatom bloom. Maximum average chlorophyll concentration in surface waters was 2.7 ± 1.2 µg 1−1 and occurred in April while surface nutrients were being depleted. In contrast to chlorophyll, maximum particulate carbon in surface waters occurred in July–August when areal productivity was highest (367–398 mg C m−2 day−1). Concurrent with spring nutrient depletion in surface waters was increasing N-deficiency in the plankton. After the spring bloom dissipated in May, particulate matter (POM) became increasingly N-deficient reaching maximum elemental C : N of > 18 during summer-fall. Profiles of the C : N ratio of POM were nearly constant with depth for individual sampling dates suggesting that the residence time of POM in the water column was short (< 1 month). While surface waters were nutrient depleted during summer stratification, nutrient concentrations of bottom waters progressively increased, presumably through the oxidation of POM sinking to the bottom (103 m). Converting the rate of oxygen depletion in bottom waters to carbon equivalents of POM suggests that 42 % of mean annual phytoplankton production in overlying waters during 1990 was mineralized in bottom waters.

Bio-optical variability across the Arctic Front

Biological and optical characterization of the Arctic Front (AF), which separates North Atlantic waters from the Greenland Sea Gyre, has not been well studied and we report, herein, the first synoptic description of bio-optical and temperature variability across the AF utilizing both shipboard (vertical and horizontal measurements utilizing the towed Undulating Oceanographic Record, UOR) and satellite (AVHRR) observations of sea surface temperature and visible band reflectances (580–680 nm). The UOR measures depth, temperature, in vivo chlorophyll fluorescence, upwelling and downwelling hemispherical (450, 488 and 550 nm) and vector (488 and 550 nm) irradiances at 10 s intervals during vertical undulations from 2 to 50 m. During a UOR tow on 19 August 1986, the AF was encountered as a sharp boundary with an abrupt change in bio-optical properties within the upper 50 m over a few miles. Temperatures increased from 5.7° to 8.4°C with the average chlorophyll concentration and diffuse attenuation coefficient [K(450)] increasing by factors of 4.1 and 1.8, respectively. Discrete samples for species composition and HPLC pigment analysis, taken within this high pigment region (150 miles in width), indicated that this near surface bloom was a mixed phytoplankton population with moderate concentrations of the coccolithophorid, Emiliania huxleyi. AVHRR visible band imagery showed a high reflectance patch adjacent to the frontal boundary which are normally associated with substantial concentration of coccolithophorids. If our observations on the magnitude and extent of the biomass are typical of the AF, it should have an important role in marine biogeochemical production in this high latitude area.

Modelling phytoplankton abundance in a small enclosed bay used for salmon farming

. A simplified mass balance model was used to predict spatially averaged nitrogen and chlorophyll concentrations in Big Glory Bay, Stewart Island, New Zealand. The bay is used for salmon fanning and during January 1989 supported a dense bloom of Heterosigma alcashiwo which killed 600t of fish. Nutrient concentrations in the bay arc affected by inputs from bottom sediments, catchment run-off, rainfall, salmon farms and water exchange with Paterson Inlet (into which Big Glory Bay drains). The nitrogen model was calibrated using field data collected on 11–27 February 1988 and successfully predicted the mean nitrogen concentration of the bay during the 1989 bloom. Nitrogen concentrations in the bay were most affected by the nitrogen concentration in Paterson Inlet, which appears to vary year-to-year as a result of the incursion into Fouveaux Strait of highly fertile oceanic water. The marginal effect of the salmon farms was to increase the mean nitrogen concentration of Big Glory Bay by about 30%. The nitrogen model was combined with a logistic phytoplankton growth model to examine the effects of nitrogen availability and hydraulic flushing on phytoplankton yields (measured as chlorophyll a). The spatially averaged chlorophyll concentration of Big Glory Bay was reliably predicted during the 1989 study, when there was strong evidence that phytoplankton were nitrogen limited. The model indicated that nitrogen inputs from the salmon farms increased the mean chlorophyll concentration of the bay by about 33%. The model was less successful in predicting chlorophyll concentrations in February 1988 when phytoplankton growth was not restricted by nitrogen, but may have been controlled by zooplankton grazing or sedimentation (which were not included in the model). The use of mo re complex three-dimensional models is advocated to describe the observed‘patchiness’in Heterosigma distribution.