The plankton community in Norwegian coastal waters—abundance, composition, spatial distribution and diel variation

The purpose of the present study was to explore the composition and variation of the pico-, nano- and micro-plankton communities in Norwegian coastal waters and Skagerrak, and the co-occurrence of bacteria and viruses. Samples were collected along three cruise transects from Jæren, Lista and Oksøy on the south coast of Norway and into the North Sea and Skagerrak. We also followed a drifting buoy for 55 h in Skagerrak in order to observe diel variations. Satellite ocean color images (SeaWiFS) of the chlorophyll a (chl a) distribution compared favorably to in situ measurements in open waters, while closer to the shore remote sensing chl a data was overestimated compared to the in situ data. Using light microscopy, we identified 49 micro- and 15 nanoplankton sized phototrophic forms as well as 40 micro- and 12 nanoplankton sized heterotrophic forms. The only picoeukaryote (0.2-2.0 μm) we identified was Resultor micron (Pedinophyceae). Along the transects a significant variation in the distribution and abundance of different plankton forms were observed, with Synechococcus spp and autotrophic picoeukaryotes as the most notable examples. There was no correlation between viruses and chl a, but between viruses and bacteria, and between viruses and some of the phytoplankton groups, especially the picoeukaryotes. Moreover, there was a negative correlation between nutrients and small viruses (Low Fluorescent Viruses) but a positive correlation between nutrients and large viruses (High Fluorescent Viruses). The abundance of autotrophic picoplankton, bacteria and viruses showed a diel variation in surface waters with higher values around noon and late at night and lower values in the evening. Synechococcus spp were found at 20 m depth 25-45 nautical miles from shore apparently forming a bloom that stretched out for more than 100 nautical miles from Skagerrak and up the south west coast of Norway. The different methods used for assessing abundance, distribution and diversity of microorganisms yielded complementary information about the plankton community. Flow cytometry enabled us to map the distribution of the smaller phytoplankton forms, bacteria and viruses in more detail than has been possible before but detection and quantification of specific forms (genus or species) still requires taxonomic skills, molecular analysis or both.     

Size-fractionated phytoplankton biomass,primary production and respiration in the Nervión–Ibaizabal estuary: A comparison with other nearshore coastal and estuarine ecosystems from the Bay of Biscay

The seasonal pattern of size-fractionated phytoplankton biomass, primary production and respiration was investigated along the longitudinal axis of the Nervión–Ibaizabal estuary (Bay of Biscay) from April 2003 to September 2004. Environmental factors influencing phytoplankton dynamics were also studied. Chlorophyll a biomass showed a longitudinal pattern of increase from the outer Abra bay to the inner estuary. On a seasonal scale, in the intermediate and inner estuary phytoplankton biomass maxima were registered in summer, the warmest and driest season, whereas in the outer bay chlorophyll a peaks occurred in May 2004, but were delayed to August 2003, likely due to a very rainy spring. Data suggest that river flow exerts a marked influence on the timing of phytoplankton biomass maxima in this estuary, decreased river flows providing a lowering of turbidity and an increase in water residence time needed for chlorophyll a to build up. Nutrient concentrations were high enough not to limit phytoplankton growth throughout the annual cycle, except silicate and occasionally phosphate in the outer bay during summer. Silicate concentration correlated positively with river flow, whereas ammonium and phosphate maximum values were generally measured in the mid-estuary, suggesting the importance of allochthonous anthropogenic sources. In the intermediate and inner estuary phytoplankton biomass was generally dominated by >8 μm size-fraction (ca. 60%), but in August 2003 <8 μm size-fraction increased its contribution in the intermediate estuary. It is argued that the lower nutrient concentrations measured in August 2003 than in August 2004 could have played a role. This is the first study in which phytoplankton primary production rates have been measured along the longitudinal axis of the Nervión–Ibaizabal estuary. Throughout the annual cycle these rates ranged from 0.001 to 3.163 g C m^?3 d^?1 and were comparable to those measured in nearby small estuaries of the Basque coast and other larger estuaries on the Bay of Biscay. Surface plankton community respiration rate maxima were measured during the spring 2004 chlorophyll a peak in the Abra bay and in summer months at the mid and inner estuary, coinciding with chlorophyll a biomass and primary production maxima. In general, respiration rates showed a positive correlation with temperature. In order to compare results from the Nervión–Ibaizabal estuary with other nearshore coastal and estuarine ecosystems within the Bay of Biscay a review of existing information on phytoplankton biomass and primary production dynamics was performed.     

Spring and summer blooms of phytoplankton (SeaWiFS/MODIS) along a ferry line in the Bay of Biscay and western English Channel

The seasonal cycle of chlorophyll concentration in the Bay of Biscay and western English Channel has been examined using satellite data (chlorophyll, sea surface temperature (SST), photosynthetically available radiation (PAR) and wind) along the line of the ferry Pride of Bilbao (Bilbao to Portsmouth). The spring phytoplankton bloom develops regularly in the oceanic region of the Bay of Biscay from mid March to the beginning of May with peak chlorophyll concentrations ranging 2–4 mg m^?3. Low wind turbulence is a major factor allowing the development of productivity pulses in the Bay of Biscay during spring. Exceptional blooms of phytoplankton take place in summer (July–August) in the western English Channel with chlorophyll concentrations as high as 40 mg m^?3. Some environmental factors (SST, wind, pressure and tide) are examined. Autumn blooms of phytoplankton (1–2 mg m^?3) are also detected in the northern Bay of Biscay, shelf-break and Celtic Sea in October. A 11 years pluri-annual synthesis of SeaWiFS satellite measurements is presented.     

On the remote monitoring of Karenia brevis blooms of the west Florida shelf

In situ surveys (1997–2002) of Karenia brevis distribution on the west Florida shelf were used to explain spectral remote sensing reflectance, chlorophyll-a concentration, and backscattering coefficient estimates derived using SeaWiFS satellite data. Two existing approaches were tested in an attempt to differentiate K. brevis blooms from other blooms or plumes. A chlorophyll-anomaly method used operationally by the National Oceanic and Atmospheric Administration (NOAA) sometimes correctly identified K. brevis blooms but also generated false positives and false negatives. The method identified approximately 1000 km² of high chlorophyll-anomalies (>1 mg m⁻³) off southwest Florida between the 10 and 50-m isobaths nearly every day from summer to late fall. Whether these patches were K. brevis blooms or not is unknown. A second method used a backscattering:chlorophyll-a ratio to identify K. brevis patches. This method separated K. brevis from other blooms using in situ optical data, but it yielded less satisfactory results with SeaWiFS data. Spectral reflectance (R_rs) estimates for K. brevis blooms, diatom blooms, and coastal river plumes are statistically similar for many cases. Large pixel size, shallow water, and imperfect algorithms distort satellite retrievals of bio-optical parameters in patchy blooms. At present, a combination of chlorophyll-a, chlorophyll-anomaly, backscattering:chlorophyll-a ratio, RGB composites, MODIS fluorescence data, as well as time-series analysis and ancillary data such as winds, currents, and sea surface temperature can improve K. brevis bloom assessments. Progress in atmospheric correction and bio-optical inversion algorithms is required to help improve capabilities to monitor K. brevis blooms from space. Further, satellite sensors with improved radiometric capabilities and temporal/spatial resolutions are also required.     

The growth dynamics of Karenia brevis within discrete blooms on the West Florida Shelf

As part of the ECOHAB: Florida Program, we studied three large blooms of the harmful bloom forming dinoflagellate Karenia brevis. These blooms formed on the West Florida Shelf during Fall of 2000 off Panama City, and during Fall 2001 and Fall 2002 off the coastline between Tampa Bay and Charlotte Harbor. We suggest that these blooms represent two different stages of development, with the 2000 and 2001 blooms in an active growth or maintenance phase and the 2002 bloom in the early bloom initiation phase. Each bloom was highly productive with vertically integrated primary production values of 0.47–0.61, 0.39–1.33 and 0.65 g C m⁻² d⁻¹ for the 2000, 2001 and 2002 K. brevis blooms, respectively. Carbon specific growth rates were low during each of these blooms with values remaining fairly uniform with depth corresponding to generation times of 3–5 days. Nitrogen assimilation by K. brevis was highest during 2001 with values ranging from 0.15 to 2.14 μmol N L⁻¹ d⁻¹ and lower generally for 2000 and 2002 (0.01–0.64 and 0.66–0.76 μmol N L⁻¹ d⁻¹ for 2000 and 2002, respectively). The highest K. brevis cell densities occurred during the 2001 bloom and ranged from 400 to 800 cells mL⁻¹. Cell densities were lower for each of the 2000 and 2002 blooms relative to those for 2001 with densities ranging from 100 to 500 cells mL⁻¹. The 2000 and 2001 blooms were dominated by K. brevis in terms of its contribution to the total chlorophyll a (chl a) pool with K. brevis accounting generally for >70% of the observed chl a. For those populations that were dominated by K. brevis (e.g. 2000 and 2001), phytoplankton C biomass (C_p,0) constituted <30% of the total particulate organic carbon (POC). However, in 2002 when diatoms and K. brevis each contributed about the same to the total chl a, C_p,0 was >72% of the POC. The fraction of the total chl a that could be attributed to K. brevis was most highly correlated with POC, chl a and salinity. Nitrogen assimilation rate and primary production were highly correlated with a greater correlation coefficient than all other comparisons.     

Remotely sensed assessment of water quality levels in the Pearl River Estuary, China

In this paper, a method of assessing water quality from satellite data is introduced. The composite pollution index (CPI) was calculated from measured chemical oxygen demand (COD) and nutrient concentration. The relationships between CPI and 240 band combinations of SeaWiFS water-leaving radiance were analyzed and the optimal band combination for estimating CPI was chosen from the 240 band combinations. An algorithm for retrieval of CPI was developed using the optimal band combination, (L₄₄₃ × L₅₁₀)/(L₄₁₂ + L₄₉₀). The CPI was estimated from atmospherically corrected SeaWiFS data by employing the algorithm. Furthermore, the CPI value range for each water quality level was determined based on data obtained from 850 samples taken in the Pearl River Estuary. The remotely sensed CPIs were then transferred to water quality levels and appropriate maps were derived. The remotely sensed water quality level maps displayed a similar distribution of levels based on in situ investigation issued by the State Ocean Administration, China. This study demonstrates that remote sensing can play an important role in water quality assessment.     

A novel technique for detection of the toxic dinoflagellate, Karenia brevis, in the Gulf of Mexico from remotely sensed ocean color data

Karenia brevis, a toxic dinoflagellate that blooms regularly in the Gulf of Mexico, frequently causes widespread ecological and economic damage and can pose a serious threat to human health. A means for detecting blooms early and monitoring existing blooms that offers high spatial and temporal resolution is desired. Between 1999 and 2001, a large bio-optical data set consisting of spectral measurements of remote-sensing reflectance (R_rs(λ)), absorption (a(λ)), and backscattering (b_b(λ)) along with chlorophyll a concentrations and K. brevis cell counts was collected on the central west Florida shelf (WFS) as part of the Ecology and Oceanography of Harmful Algal Blooms (ECOHAB) and Hyperspectral Coastal Ocean Dynamics Experiment (HyCODE) programs. Reflectance model simulations indicate that absorption due to cellular pigmentation is not responsible for the factor of ∼3–4 decrease observed in R_rs(λ) for waters containing greater than 10⁴ cells l⁻¹ of K. brevis. Instead, particulate backscattering is responsible for this decreased reflectivity. Measured particulate backscattering coefficients were significantly lower when K. brevis concentrations exceeded 10⁴ cells l⁻¹ compared to values measured in high-chlorophyll (>1.5 mg m⁻³), diatom-dominated waters containing fewer than 10⁴ cells l⁻¹ of K. brevis. A classification technique for detecting high-chlorophyll, low-backscattering K. brevis blooms is developed. In addition, a method for quantifying chlorophyll concentrations in positively flagged pixels using fluorescence line height (FLH) data obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) is introduced. Both techniques are successfully applied to Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and MODIS data acquired in late August 2001 and validated using in situ K. brevis cell concentrations.     

Investigation of the temporal relation of remotely sensed coastal water quality with GIS modelled upstream soil erosion

Hydrological processes at the river basin influence the quality of downstream water bodies by controlling the loads of nutrients and suspended solids. Although their monitoring is important for social, economic and environmental reasons, in‐situ measurements are too expensive and thus too sparse to describe their relations. The aim of this study is to investigate the temporal relations of soil erosion in the upstream part of river basins with water quality characteristics in the downstream coastal zone, using satellite remote sensing and GIS modelling. Data from satellite missions of MODIS, SRTM and TRMM were used to describe the soil erosion factors of the Universal Soil Loss Equation in three river basins, and MERIS satellite data was used to estimate chlorophyll‐a and total suspended matter concentrations in the coastal zone of northwest Aegean Sea in Greece, where the rivers discharge. The resulting time series showed an average correlation of upstream rainfall with downstream water quality, which increased when soil erosion was introduced. Higher correlations were observed with the use of a time lag, revealing a variable delay between the three test sites. Lower correlation coefficients were observed for chlorophyll‐a, due to the sensitivity of algae to environmental conditions. The use of free of charge satellite data and easy to operate GIS models renders the findings of this work useful for coastal zone management bodies, in order to help increase aquaculture productivity, predict algal blooms and predict siltation of ports. Copyright © 2014 John Wiley & Sons, Ltd.     

Turbulent mixing in fine-scale phytoplankton layers: Observations and inferences of layer dynamics

Turbulence measurements in fine-scale phytoplankton layers (∼1 to ∼10 m) in the Gulf of Aqaba (Red Sea) were used to evaluate mechanisms of layer formation, maintenance, and breakdown. Simultaneous profiles of chlorophyll a (Chl a) fluorescence and temperature microstructure were measured in the upper 40 m of a 430 m water column over a 16-d period, using a Self Contained Autonomous MicroProfiler (SCAMP). Layers of concentrated phytoplankton were identified in 95 of the 456 profiles. The layers were situated in density stratified regions between 15 and 38 m depth and were characterized by intensities of 0.1 to 0.35 μg Chl a L⁻¹ (as much as two times background concentrations) and an average thickness of 10 m. We show that turbulent mixing and isopycnal displacements associated with internal waves modulated the thickness of the layers. Variations in mixing rates within layers were connected to the vertical structure of the stratified turbulence and the stage of layer development. The breakdown of a persistent phytoplankton layer was tied to strong turbulent mixing at the base of the surface mixed layer, which encroached on the layer from above. Hydrographic observations and scaling analysis suggest that the layers most likely formed in horizontal intrusions from the adjacent coastal region. The cross-shore propagation of phytoplankton-rich intrusions may have important implications for the trophic state of offshore planktonic communities.     

Short-term and seasonal variation in metabolic balance in Liverpool Bay

Regions of freshwater influence (ROFIs) are dynamic areas within the coastal seas that experience cycles of stability driven by density gradients and the spring-neap tidal cycle. As a result, pulses of biological production may occur on a more frequent timescale than the classic seasonal cycle. Net community production (NCP) rates and chlorophyll a concentration are presented from a site within the ROFI of Liverpool Bay and compared to similar measurements made at a site outside the ROFI during 2009. The influence of water column stability on biological production in the ROFI was also investigated using high-frequency observations from a Cefas Smartbuoy. Both sites were autotrophic from spring to autumn before becoming heterotrophic over winter. NCP at the inshore site was estimated to range from 30.8 to 50.4 gC m⁻² year⁻¹. A linear relationship detected between chlorophyll a and NCP from both sites was used to estimate metabolic balance over 1 year at the ROFI site using high-resolution chlorophyll a concentrations from the Smartbuoy but was found to poorly replicate NCP rates compared to those derived from dissolved oxygen fluxes. There was no clear biological response to periods of stratification within the ROFI, and it is proposed that changes in light attenuation in the Liverpool Bay ROFI, driven not only by stratification but also by fluctuations in riverine sediment load, most likely play an important role in controlling phytoplankton growth in this region.     

Nitrogen over enrichment in subtropical Pearl River estuarine coastal waters: Possible causes and consequences

Hong Kong is surrounded by estuarine, coastal and oceanic waters. In this study, monthly averages over a 10 year time series of salinity, temperature, chlorophyll a (chl a), dissolved oxygen (DO), dissolved inorganic nitrogen (DIN), silicate (SiO₄) and orthophosphate (PO₄) at three representative stations around Hong Kong were used to examine if excess nitrogen in estuarine influenced waters is due to P limitation. The monthly distribution clearly shows the dominant influence of the seasonal change in river discharge in the Pearl River estuary and adjacent coastal waters. In winter, the river discharge is small and more oceanic waters are dominant and as a result, salinity is high, and chlorophyll and nutrients are low. In summer, when the river discharge is high, salinity decreases and nutrients increase. DIN is very high, reaching 100 μM in the estuary. This indicates over enrichment of nitrogen relative to P and consequently there is an excess of N in coastal waters of Hong Kong. P remains low (∼1 μM) and can potentially limit both phytoplankton biomass and N utilization which was demonstrated in field incubation experiments. P limitation would result in excess N being left in the estuarine influenced waters south of Hong Kong. Phosphate concentration is lower in the Pearl River estuary than in many other eutrophied estuaries. Therefore, this relatively low PO₄ concentration should be a significant factor limiting a further increase in the magnitude of algal biomass and in the degree of eutrophication in the Pearl River estuary. The export of the excess N offshore into the northern South China Sea may result in an increase in the size of the region that is P limited in summer.     

Seasonal contribution of living phytoplankton carbon to vertical fluxes in a coastal upwelling system (Ría de Vigo, NW Spain)

The aim of this study is to explore the contribution of living phytoplankton carbon to vertical fluxes in a coastal upwelling system as a key piece to understand the coupling between primary production in the photic layer and the transfer mechanisms of the organic material from the photic zone. Between April 2004 and January 2005, five campaigns were carried out in the Ría de Vigo (NW Iberian Peninsula) covering the most representative oceanographic conditions for this region. Measurements of particulate organic carbon (POC), chlorophyll-a (chl a), phaeopigments (phaeo), and identification of phytoplankton species were performed on the water column samples and on the organic material collected in sediment traps.The POC fluxes measured by the sediment traps presented no seasonal variation along the studied period ranging around a mean annual value of 1085±365 mg m⁻² d⁻¹, in the upper range of the previously reported values for other coastal systems. The fact that higher POC fluxes were registered during autumn and winter, when primary production rates were at their minimum levels points to a dominant contribution of organic carbon from resuspended sediments on the trap collected material. On the contrary, fluxes of living phytoplankton carbon (C_phyto) and chl a clearly presented a seasonal trend with maximum values during summer upwelling (546 mg m⁻² d⁻¹ and 22 mg chl a m⁻² d⁻¹, respectively) and minimum values during winter (22 mg m⁻² d⁻¹ and 0.1 mg chl a m⁻² d⁻¹, respectively). The contribution of C_phyto to the vertical flux of POC ranged between 2% and 49% in response to the pelagic phytoplankton community structure. Higher values of C_phyto fluxes were registered under upwelling conditions which favour the dominance of large chain-forming diatoms (Asterionellopsis glacialis and Detonula pumila) that were rapidly transferred to the sediments. By contrast, C_phyto fluxes decreased during the summer stratification associated with a pelagic phytoplankton community dominated by single-cell diatoms and flagellates. Minimal C_phyto fluxes were observed during the winter mixing conditions, when the presence of the benthic specie Paralia sulcata in the water column also points toward strong sediment resuspension.     

Spatial and temporal variability of phytoplankton and environmental factors in a temperate estuary of South America (Atlantic coast,Argentina)

A spatial and temporal study on data collected along the longitudinal gradient of the Principal Channel of Bahía Blanca estuary, Argentina, was carried out during 1992–1993. At nine stations, phytoplankton abundance, chlorophyll a (Chl-a) concentration, inorganic nutrient levels, Secchi disk depth, euphotic depth:mixing depth ratio (Z_eu:Z_m), salinity and temperature were recorded. Phytoplankton abundance, Chl-a concentration and nutrient levels decreased towards the outer zone of the estuary. The inner zone (stations 1 and 2), which was characterized by high turbidity, high nutrient concentrations and high Z_eu:Z_m (>0.16, [critical mixing ratio]), registered the highest phytoplankton abundance and Chl-a concentrations. Temporal variability of data was also noteworthy in this zone. The highest biomass values thus corresponded to June, July, August and the beginning of spring (18 μg Chl-a L⁻¹ and 9×10⁶ cells L⁻¹) concomitantly with a diatom bloom. In the middle zone (stations 3–6), a strong phytoplankton biomass decrease was observed and it coincided with both deep-mixed depths and low Z_eu:Z_m (<0.16). The outer zone (stations 7–9), which was characterized by low phytoplankton biomass values and low nutrient levels all along the year, was the area mostly influenced by waters from the adjacent continental shelf. In view of the above, it can be concluded that the most important primary production in the Bahía Blanca would be produced in the shallow inner zone during winter, being the spatial reach of the phytoplankton biomass principally limited to estuarine waters. Presumably, less than 5% of such biomass may reach the coastal area of the estuary.     

Temporal and spatial variations of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) in the East China Sea and the Yellow Sea

Temporal and spatial distributions of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) were determined in the East China Sea and the Yellow Sea during June-July, 2006 and January-February, 2007. The concentrations of DMS and total DMSP in surface water in the study area were 5.64 (1.79-12.24) and 28.25 (13.98-44.93) nmol L⁻¹ in summer, and were 1.79 (1.02-3.51) and 11.01 (6.90-17.98) nmol L⁻¹ in winter, respectively. The distributions of DMS and DMSP in the study area were obviously influenced by the Yangtze River effluent and the Kuroshio water. Even under highly variable hydrographic conditions, a significant relationship was observed between DMS and chlorophyll a concentrations in summer as well as in winter, suggesting that phytoplankton biomass might play an important role in controlling DMS distribution in the study area. The summer ratios of DMS/chlorophyll a and DMSP/chlorophyll a were approximately twofold higher than winter values, corresponding with the temporal variation in phytoplankton community structure between summer and winter. The sea-to-air fluxes of DMS were estimated to be 5.32 and 11.92 μmol m⁻² d⁻¹ using the equations of Liss and Merlivat (1986) and Wanninkhof (1992), respectively.     

Hydrodynamic accumulation of Karenia off the west coast of Florida

Blooms of the toxic dinoflagellates, Karenia spp. occur nearly annually in the eastern Gulf of Mexico with cell abundances typically >10⁵ cells L⁻¹. Thermal and ocean color satellite imagery shows sea surface temperature patterns indicative of upwelling events and the concentration of chlorophyll at fronts along the west Florida continental shelf. Daily cell counts of Karenia show greater increases in cell concentrations at fronts than can be explained by Karenia's maximum specific growth rate. This is observed in satellite images as up to a 10-fold greater increase in chlorophyll biomass over 1–2 d periods than can be explained by in situ growth. In this study, we propose a model that explains why surface blooms of Karenia may develop even when nutrients on the west Florida shelf are low. In the summer, northward winds produce a net flow east and southeast bringing water and nutrients from the Mississippi River plume onto the west Florida shelf at depths of 20–50 m. This water mass supplies utilizable inorganic and organic forms of nitrogen that promote the growth of Karenia to pre-bloom concentrations in sub-surface waters in the mid-shelf region. In the fall, a change to upwelling favorable winds produces onshore transport. This transport, coupled with the swimming behavior of Karenia, leads to physical accumulation at frontal regions near the coast, resulting in fall blooms. Strong thermal fronts during the winter provide a mechanism for re-intensification of the blooms, if Karenia cells are located north of the fronts. This conceptual model leads to testable hypotheses on bloom development throughout the Gulf of Mexico.     

Variations in light absorption properties during a phytoplankton bloom in the Pearl River estuary

From 15 to 28 August in 2007, a Chaetoceros socialis bloom was detected in the Pearl River Estuary water with chlorophyll a concentration (Chl a) up to 30 mg m⁻³ and cell density up to 10⁶ cells L⁻¹. Time series of bio-optical measurements was obtained at a single site (114.29°E, 22.06°N) with the mooring of marine optical buoy. Light absorption properties of seawater experienced large variability throughout the algal bloom. Absorption by colored dissolved organic matter (CDOM) was one of the dominant optical components of the light absorption (30–70%) especially for pre- and post-bloom waters, and it tended to decrease with Chl a during the algal bloom. Absorption by phytoplankton was another dominant optical component (18–50%) and increased rapidly with Chl a. Phytoplankton and accompanying material played dominant roles in light absorption as indicated by the relationship between absorption coefficient and Chl a. At high pigment concentrations, water samples showed significantly lower specific phytoplankton absorption, compared with pre- and post-bloom conditions, with the specific phytoplankton concentration at 443 nm varied between 0.011 and 0.022 m² mg⁻¹ and that at 676 nm between 0.007 and 0.018 m² mg⁻¹; small values of blue-to-red ratio of phytoplankton were also observed. These lower values were associated with variations in phytoplankton size structure. Spectral variability of phytoplankton absorption and total absorption (not including the fixed background absorption by pure water itself) could be expressed as simple linear functions linking absorption at one wavelength to the absorption at the other wavelengths, with the slope of the relationship changing with wavelength. The absorption coefficients by non-algal particles and CDOM follow the general exponential functions with remarkably limited variability in the exponent with means of 0.0105 and 0.0166 nm⁻¹, respectively. These spectral dependencies of absorption coefficients provide useful information for retrieving inherent optical properties from reflectance data in a remote-sensing context.     

DMSP and DMS in coastal fast ice and under-ice water of Lützow-Holm Bay, eastern Antarctica

The combined concentration of total dimethylsulfoniopropionate and dimethylsulfide (DMSP+DMS) were measured in Antarctic fast ice on the coast of Lützow-Holm Bay, eastern Antarctica. High bulk-ice DMSP+DMS and chlorophyll a concentrations were found at the bottom of the sea ice, and these concentrations were higher than those in the under-ice water. The bulk-ice DMSP+DMS and chlorophyll a concentrations were highly correlated (r²=0.68, P<0.001), suggesting that the high bulk-ice DMSP+DMS concentrations were caused mainly by the presence of algae assemblages in the ice. The calculated brine DMSP+DMS concentrations were as high as 1100 nM in the bottom ice layer, and the vertical profile patterns of brine DMSP+DMS concentrations were almost the same as for the bulk ice, mainly because of the small amount of variability in the vertical brine volume fraction. DMSP+DMS and chlorophyll a concentrations in the under-ice water increased, whereas the salinity of the under-ice water decreased, during the sampling period. These results reflect the supply of freshwater containing high levels of DMSP+DMS to the water just under the ice as the ice melted. These results suggest that sea-ice melting could be important to sulfur cycling in coastal ice-covered regions of the polar oceans.     

Contrasting phytoplankton community structure and associated light absorption characteristics of the western Bay of Bengal

Absorption spectra, particulate pigments, and hydrochemical constituents were measured in the western Bay of Bengal (BoB) during July-August 2010 when influence of river discharge is at peak. Chromophoric dissolved organic matter (CDOM) absorption coefficient (a_CDOM(440)) displayed a significant inverse linear relationship with salinity in the surface waters implying conservative mixing of marine and terrestrial end members. The northern part of the study area is influenced by discharge from the river Ganga and a dominant terrestrial CDOM signal is seen. The southern part receives discharge from peninsular rivers with corresponding signals of higher CDOM than the linear model would indicate and higher UV-specific absorption coefficient (SUVA) indicating more aged and humified DOM. Lower contribution of CDOM to total non-water absorption and higher phytoplankton biomass (chlorophyll a absorption coefficient, a_ph(440)) but lower chlorophyll a specific phytoplankton absorption coefficient (a _ph ^* (440)) characterize the northern part, compared to the southern part. Chlorophyll b had a distinct linear relationship with chlorophyll a in the latter. The size index (SI) indicated dominance of microphytoplankton in the northern and nano and picophytoplankton in the southern parts. Chlorophyll a is significantly related to a _ph ^* (440) by an inverse power model in the northern part but by an inverse linear model in the southern part. Our study suggests that knowledge of the phytoplankton community structure is essential to improve chlorophyll a algorithm in the coastal Bay of Bengal.