Vertical Profiles Of Chlorophyll Research Articles

Distinct habitat and biogeochemical properties of low‐oxygen‐adapted tropical oceanic phytoplankton

AbstractWe use data collected by Biogeochemical Argo (BGC‐Argo) float, over a 5‐year period (2016–2021), to study the dynamics of a unique low‐oxygen‐adapted phytoplanktonic community in the eastern tropical North Pacific. We isolate this community using a model that partitions vertical profiles of chlorophyll a (Chl a) and particulate backscattering into the contributions of three communities of phytoplankton: C1, the community in the mixed‐layer; C2, at the deep Chl a maximum; and C3, in low‐oxygen waters at the base of the euphotic zone. We find that C3 has a similar chl‐specific particulate backscattering to C2, both lower than C1. C2 and C3 contribute significantly to integrated stocks of Chl a, both at around 41%, and both around 30% of integrated particulate backscattering (after removing a background signal attributed to nonalgal particles). Found at depths of around 100 m, the peak biomass of C3 is lower than C2 (located at around 60 m), and yet, C3 makes similar contributions to integrated stocks, because it has a broader peak than C2. In relation to C1 and C2, C3 thrives in a lower temperature, higher density, lower light, lower oxygen, and higher saline habitat. This work illustrates how BGC‐Argo floats, in combination with simple conceptual models, can be used to observe the dynamics of unique communities of phytoplankton in extreme environments. The projected climate‐driven changes in oxygen minimum zones add urgency to understand the vulnerabilities of these communities both in terms of stocks and composition.

Improved Perceptron of Subsurface Chlorophyll Maxima by a Deep Neural Network: A Case Study with BGC-Argo Float Data in the Northwestern Pacific Ocean

Subsurface chlorophyll maxima (SCMs), commonly occurring beneath the surface mixed layer in coastal seas and open oceans, account for main changes in depth-integrated primary production and hence significantly contribute to the global carbon cycle. To fill the gap of previous methods (in situ measurement, remote sensing, and the extrapolating function based on surface-ocean data) for obtaining SCM characteristics (intensity, depth, and thickness), we developed an improved deep neural network (IDNN) model using a Gaussian radial basis activation function to retrieve the vertical profile of chlorophyll a concentration (Chl a) and associated SCM characteristics from surface-ocean data. The annually averaged SCM depth was further incorporated into the bias term and the Gaussian activation function to improve the estimation accuracy of the IDNN model. Based on the Biogeochemical-Argo (BGC-Argo) data acquired for three regions in the northwestern Pacific Ocean, vertical Chl a profiles produced by our improved DNN model using sea surface Chl a and sea surface temperature (SST) were in good agreement with the observations, especially in regions with low surface Chl a. Compared to other neural-network-based models with one hidden layer and a sigmoid activation function, the IDNN model retrieved vertical Chl a profiles well in more eutrophic subpolar regions. Furthermore, the application of the IDNN model to infer vertical Chl a profiles from remote-sensing information was validated in the northwestern Pacific Ocean.

A novel method to simulate the 3D chlorophyll distribution in marine oligotrophic waters

A 3D advection-diffusion-reaction model is proposed to investigate the abundance of phytoplankton in a difficult-to-access ecosystem such as the Gulf of Sirte (southern Mediterranean Sea) characterized by oligotrophic waters. The model exploits experimentally measured environmental variables to reproduce the dynamics of four populations that dominate phytoplankton community in the studied area: Synechococcus, Prochlorococcus HL, Prochlorococcus LL and picoeukaryotes. The theoretical results obtained for phytoplankton abundances are converted into chl-a and Dvchl-a concentrations, and the simulated vertical chlorophyll profiles are compared to the corresponding experimentally acquired data during the MedSudMed-08 oceanographic survey. Non parametric tests showed that the 3D model successfully simulates the spatial distribution of chlorophyll in most of the Gulf of Sirte. Statistical analysis indicates that about 80% of the simulated chlorophyll vertical profiles are not significantly different from the experimental data. This model is also a useful tool to predict chlorophyll distributions in hard-to-access areas, where experimental data cannot be collected. Moreover, the model could be used to predict the effects of global warming on phytoplankton dynamics and primary production in marine ecosystems.

Spatial and Temporal Vertical Distribution of Chlorophyll in Relation to Submarine Wastewater Effluent Discharges

The vertical distribution of chlorophyll in coastal waters is influenced by a combination of the hydrodynamic environment and different biotic and abiotic processes. The spatial and temporal occurrences of chlorophyll profiles provide a good representation of the changes in the marine environment. The majority of studies in the Adriatic Sea have so far been conducted in areas unaffected by anthropogenic pressure. Our study site is located near two marine outfalls, which are part of the public sewage system. This study aims to characterize the chlorophyll vertical distribution and describe its variability based on the stratification conditions and the presence of a wastewater effluent plume. Based on these conditions, we identified three characteristic scenarios/types of chlorophyll profiles. The first one occurs when the vertical mixing of the water column creates the upwelling of chlorophyll and nutrients to the upper part of the water column. The second and third scenarios occur during stratified conditions and differ by the extent of the effluent plume intrusion. Using modern fluorescence techniques, we identified and described three different vertical chlorophyll profiles, characterizing them according to their physical and biological parameters and processes. For cases with a visible effluent intrusion, we confirmed the importance of the pycnocline formation in keeping the effluent below and maintaining the higher water quality status at the top of the water column.

Functional resilience of PSII, vertical distribution and ecosystem-level estimates of subsurface microphytobenthos in estuarine tidal flats

Most studies on sediment-inhabiting microphytobenthos are based on the biomass present on the surface layers of intertidal flats. However, large amounts of microalgal biomass are known to exist below the surface. This study tested the role of subsurface microalgal biomass as a potential source of photosynthetically active cells for the biofilm on the surface. The resilience of buried cells was evaluated by exposing samples from various depths to surface conditions and investigating the recovery of photosynthetic activity. Additionally, vertical migration by subsurface epipelic diatoms was followed at sub-millimeter scales to evaluate its role for transporting cells to the vicinity of the sediment surface. Finally the relative importance of subsurface microalgal biomass was assessed by estimating the proportion of subsurface:surface biomass for different types of sediments from the Ria de Aveiro. Vertical profiles of chlorophyll a, 10 cm-deep, were measured on samples from three intertidal sites, representative of the range of sediment characteristics found in this estuary. The ratio of total biomass to surface biomass (‘subsurface biomass fraction’) based on total biomass (0–10 cm depth interval; Csub, total) and on viable biomass (between the surface and the maximum depth with significant photosynthetic recovery; Csub, viable). The experiments showed that buried cells were able to recover photosynthetic activity within 1.5–3 h of light exposure, with the rate of recovery being dependent on depth and type of sediment. Furthermore, subsurface vertical migration was found to enable motile cells to reach the surface from layers deeper than 1 mm within a low tide period. Overall, the results showed that surface biomass (0–0.5 cm) only accounted for one fifth to one third of the total biomass present between the surface and 10 cm, and that the amount of subsurface viable biomass reached 2–3 times the biomass present at the surface. Applying the estimates of Csub, total and Csub, viable to the whole intertidal area of the Ria de Aveiro, spatially-weighted averages for subsurface biomass fractions were found to reach 3.8 and 2.1 respectively.

Analytical solution of the nitracline with the evolution of subsurface chlorophyll maximum in stratified water columns

Abstract. In a stratified water column, the nitracline is a layer where the nitrate concentration increases below the nutrient-depleted upper layer, exhibiting a strong vertical gradient in the euphotic zone. The subsurface chlorophyll maximum layer (SCML) forms near the bottom of the euphotic zone, acting as a trap to diminish the upward nutrient supply. Depth and steepness of the nitracline are important measurable parameters related to the vertical transport of nitrate into the euphotic zone. The correlation between the SCML and the nitracline has been widely reported in the literature, but the analytic solution for the relationship between them is not well established. By incorporating a piecewise function for the approximate Gaussian vertical profile of chlorophyll, we derive analytical solutions of a specified nutrient–phytoplankton model. The model is well suited to explain basic dependencies between a nitracline and an SCML. The analytical solution shows that the nitracline depth is deeper than the depth of the SCML, shoaling with an increase in the light attenuation coefficient and with a decrease in surface light intensity. The inverse proportional relationship between the light level at the nitracline depth and the maximum rate of new primary production is derived. Analytic solutions also show that a thinner SCML corresponds to a steeper nitracline. The nitracline steepness is positively related to the light attenuation coefficient but independent of surface light intensity. The derived equations of the nitracline in relation to the SCML provide further insight into the important role of the nitracline in marine pelagic ecosystems.

Measurement of water column primary production using photosynthesis-irradiance relations for surface phytoplankton, the vertical chlorophyll profile, and underwater light intensity

A method has been developed to measure water column integrated primary production (PPint) in the water column using photosynthesis-irradiance relations for surface phytoplankton, the vertical profile of chlorophyll a concentration, and the underwater light intensity. Good correlation has been found for the results calculated with this method and light dependences in situ. The advantages of this method are the independence of PPint calculation from CTD profiling and water sampling, and thus optimization (reduction) of the station working time.

Fitting vertical chlorophyll profiles in the California Current using two Gaussian curves

Vertical distribution of chlorophyll a (Chla) in the euphotic zone has been described and modeled by a Gaussian curve representing those cases where there is a single Chla maximum (surface or subsurface). This has led to the rejection of profiles with more than one chlorophyll peak, resulting in the removal of up to 18% of the profiles. The aim of this article is to propose an alternative equation to fit vertical Chla profiles, including those with two maxima in the euphotic zone. A total of 315 vertical profiles of Chla from the 2011 CALCOFI four-cruise series in the California Current were evaluated. Since Chla profiles show various shapes, it was decided to try four models: single Gaussian curve (single maximum) with and without gradient and double Gaussian curves (two maxima) also with and without gradient, which were fitted using Genetic Algorithms. The model that fitted Chla profiles best was the double maximum with gradient model, which is the one with the lowest root mean square error and was able to fit almost all Chla profiles in the CalCoFi area. This model has the ability to fit the vertical chlorophyll profiles with one or two maxima, including those that occur with a gradient with depth. Although the models were tested using data from the California Current, we recommend using the model in other ocean regions where double chlorophyll maximums occur.

Steady-state solutions for subsurface chlorophyll maximum in stratified water columns with a bell-shaped vertical profile of chlorophyll

Abstract. A bell-shaped vertical profile of chlorophyll a (Chl a) concentration, conventionally referred to as a subsurface chlorophyll maximum (SCM) phenomenon, has frequently been observed in stratified oceans and lakes. This profile is assumed to be a general Gaussian distribution in this study. By substituting the general Gaussian function into ecosystem dynamical equations, the steady-state solutions for SCM characteristics (i.e., SCM layer depth, thickness, and intensity) in various scenarios are derived. These solutions indicate that (1) the maximum concentration of Chl a occurs at or below the depth of maximum growth rates of phytoplankton located at the transition from nutrient limitation to light limitation, and the depth of SCM layer deepens logarithmically with an increase in surface light intensity; (2) thickness and intensity of the SCM layer are mainly affected by nutrient supply, but independent of surface light intensity; and (3) intensity of the SCM layer is proportional to the diffusive flux of nutrients from below, which becomes stronger as a result of this layer being shrunk by a higher light attenuation coefficient or a larger sinking velocity of phytoplankton. In addition, the limitation and potential application of the analytical solutions are also presented.

Modelling the inherent optical properties and estimating the constituents׳ concentrations in turbid and eutrophic waters

Retrieval of the inherent optical properties and estimation of the constituents׳ concentrations from satellite ocean colour data in turbid and eutrophic waters are important as these products provide innovative opportunities for the study of biological and biogeochemical properties in such optically complex waters. This paper intends to develop models to retrieve absorption coefficients of phytoplankton, suspended sediments and coloured dissolved organic matter and describe vertical profiles of chlorophyll and suspended sediments from satellite ocean colour data. These models make use of the relationships between remote sensing reflectance ratios Rrs (555)/Rrs (443) and Rrs (620)/Rrs (490) versus aph (443) and aph (555), and acdom (443), and ad (443) to derive the model parameters. Validation with the in-situ data obtained from coastal waters around India and other regional waters (e.g., NASA bio-Optical Marine Algorithm Data-Set, NOMAD) shows that the new models are more accurate in terms of producing the spectral absorption coefficients (aph, ad, acdom across the entire visible wavelengths 400–700nm) in a wide variety of waters. Further comparison with existing models shows advantage of the new models that have important implications for remote sensing of turbid coastal and eutrophic waters. The retrieved absorption coefficients of phytoplankton and suspended sediments (non-algal matter) are also found to relate better to chlorophyll and total suspended sediments. Taking advantages of this, we derive models to determine and describe the vertical profiles of chlorophyll and suspended sediment concentrations along the depth. The model parameters are derived empirically. These new parameterizations show potential in estimating the vertical profiles of chlorophyll and suspended sediments with good accuracy. These results suggest robustness and suitability of the new models for studying the ecologically important components of optically complex turbid and eutrophic waters using remote sensing data.

이른 여름 동중국해 대륙사면의 해양환경과 소형 식물플랑크톤 군집의 연직분포 특성

Studies of the distribution of micro-phytoplankton community and chlorophyll a concentration have focused on the vertical profiles of marine environmental factors such as water temperature, salinity, sigma-t, light intensity, and dissolved oxygen in the continental slope on the east parts of East China Sea in the early summer of 2009. Water temperature showed a gradual reduction according to the depth. While the salinity was low in the surface layer showing a mixed down to the relatively subsurface layer, it was increased with an increase in the depth at the middle and bottom layers showing a maximum value at 150~200 m followed by a decreasing aspect afterwards, although the change was not large. The change of sigma-t was governed by the water temperature, and gradually increased in the surface layer with an increase in the depth, showing a value higher than in the surface layer by about 3 at the bottom layer. Although the intensity of light was exponential reduced in the surface layer, the compensation depth was located at the depth of about 80m. The vertical profiles of chlorophyll a concentration was governed by the intensity rather than the changes in water temperature or salinity, exhibiting a maximum value at the compensation depth corresponding to 1% in the surface light intensity. The micro-phytoplankton communities consisted of 56 genera 103 species showing a relatively variety, while the standing crop was also changed to 112.0~470.0 cells/L in the pelagic environment, showing a maximum chlorophyll a concentration. Although a variety of dominant species appear at low dominance without dominant species appearing with a right-wing point in the phytoplankton communities, the silicoflagellate, Otactis otonaris at the station A and the dominance of 26% due to Leptocylindrus mediterraneus at the station C have been judged to be unusual. For community analysis of infinitesimal creatures such as phytoplankton of oligotrophic waters through the present study, ecology studies through vertical sample collection agreeing with the results of continuous observation such as identification of vertical distribution in a marine environment or of maximum chlorophyll layers have been considered rather than a survey method with intervals of a given depth such as surface, subsurface, middle and bottom layers.

Phytoplankton phenology and production around Iceland and Faroes

Phytoplankton phenology and primary production were examined in the Iceland–Faroe region through synthesis of all available data, both in situ and remotely sensed. In the Arctic water, the early onset of stratification in spring gave rise to the rapid shallowing of the mixed layer and triggered the earlier spring bloom north of Iceland, whereas the weakly stratified water-column in the Atlantic water and associated deep mixed layer delayed the spring bloom south of Iceland. The protocol (Nearest Neighbor Method, NNM) developed by Platt et al. (2008) was used to estimate the daily, water-column primary production from ocean color data. The key element of the procedure is an archived database, including (in this implementation) 505 sets of parameters of photosynthetic–light curves and 197 vertical profiles of chlorophyll around Iceland–Faroe region. The spatial structure in the climatology of annual primary production determined in this way was consistent with observations made by the simulated in situ method using ships as a platform, but, inevitably, the fields produced from the remotely sensed data were smoother. The annual primary production estimated by the NNM method overestimates the (much more sparse) data for in situ production by 50% on average. We examined the relative errors in the estimation of primary production that would arise from ignorance of the non-uniformity in the biomass profile. The vertically uniform model tended to underestimate the annual primary production by about 36% compared with the non-uniform model in a spectral calculation.

夏季西南印度洋叶绿素a分布特征

分析了2011年1月西南印度洋叶绿素a的分布特征及其粒级结构,并结合水动力学环境和营养盐数据探讨了其主要影响因素。结果表明,西南印度洋副热带涡流(IOSG)区表层叶绿素a浓度较低,不超过0.07 mg/m³,次表层叶绿素a浓度最大值所在水层较深,超过100 m;副热带聚集区(SCZ)表层叶绿素a浓度较高(0.164-0.247 mg/m³),次表层叶绿素a浓度最大值出现在50-70 m层。硝酸盐是该海区浮游植物生长的主要限制因素。微微型(pico)粒级的浮游植物占绝对优势,所有站位其对总叶绿素a的平均贡献率为71.1%,微型(nano)粒级次之(24.2%),小型(net)粒级所占比例最小(4.7%),其中IOSG区pico粒级对总叶绿素a的平均贡献率为77.9%,SCZ的pico粒级对总叶绿素a的平均贡献率为66.7%。IOSG区和SCZ海区之间水动力学环境的不同,可能导致了这两个海区叶绿素a的分布特征及粒级结构的较大差异。;An investigation of the distribution and size composition of chlorophyll a concentration was carried out in the Southwestern Indian Ocean (47-70°E, 25-40°S) in January 2011, and factors that might affect the distribution and size composition were discussed. Fluorometric method was used to obtain total and size-fractioned chlorophyll a concentrations. The survey was carried out during the austral summer, and there were 6 stations altogether, two of which located in the region of the south Indian Ocean Subtropical Gyre (IOSG) and the other four located in the subtropical convergence zone (SCZ). Water samples were collected from 13 layers in 0-200m column of each station and the vertical profiles of chlorophyll a, nutrients (NO₃^- 、PO₄^3- and SiO₃^2-), temperature and salinity were obtained. The objective of this study was to investigate the spatial distribution and size fraction of chlorophyll a concentration and to relate them to environmental factors. The results showed that the surface chlorophyll a concentration was low and did not exceed 0.07mg/m³, and the depth of the subsurface chlorophyll a maximum was deep, at approximately 100m and sometimes reaching as deep as 150m in the region of IOSG; However, in the SCZ, the surface chlorophyll a concentration was higher (0.164-0.247 mg/m³), and the subsurface chlorophyll a maximum layer occurred between 50-70m. In the region of IOSG, phytoplankton growth is limited by nutrients supply, while the frontal system formed by the Agulhas Return Current (ARC) and the Subtropical Convergence (STC) is a region of intense mesoscale activity presenting enhanced levels of biological production and chlorophyll a concentration. Based on the discussion of the factors affecting the distribution of phytoplankton, it appeared that NO₃^- was the main factor that limited the growth of phytoplankton in the southwestern Indian Ocean, and PO₄^3- could be the main limiting factor in some areas. Size-fractioned chlorophyll a concentration clearly showed that the study area was a typical oligotrophic open ocean. The picophytoplankton (0.2-2μm) chlorophyll a always dominated, representing 71.1% of the total chlorophyll a concentration on average, while the average contribution of nanophytoplankton (2-20μm) and microphytoplankton (>20μm) chlorophyll a to total chlorophyll a concentration were 24.2% and 4.7% respectively. The dominance of picophytoplankton was resulted from the low nutrient concentration environment because picophytoplankton were more competent over the microphytoplankton and nanophytoplankton under oligotrophic condition. There were some differences on the contribution of picophytoplankton to total chlorophyll a concentration between the regions of IOSG and SCZ, which were 77.9% and 66.7% respectively. The differences of the hydrographical characteristics, which in turn influenced the nutrient dynamics in the region of IOSG and SCZ, might lead to the significant difference of the spatial distribution and size composition of chlorophyll a concentration between these two regions.

A Bayesian approach for estimating vertical chlorophyll profiles from satellite remote sensing: proof-of-concept

Abstract Williamson, R., Field, J. G., Shillington, F. A., Jarre, A., and Potgieter, A. 2011. A Bayesian approach for estimating vertical chlorophyll profiles from satellite remote sensing: proof-of-concept. – ICES Journal of Marine Science, 68: 792–799. A proof-of-concept demonstration is presented using a novel method for estimating vertical distributions of chlorophyll a (Chl a) from archives of data from ships, combined with remotely sensed data of sea surface temperature, surface Chl a, and wind (U and V vectors) from satellites. Our study area has contrasting hydrographic regimes that include the dynamic southern Benguela upwelling system and the stratified waters of the Agulhas Bank. Cluster analysis is used to identify “typical” Chl a profiles from an archive of profiles recorded in 2002–2008. Bayesian networks were then used to relate characteristic profiles to remotely sensed surface features, subregions, seasons, and depths. The proposed method could be used to predict daily Chl a profiles for each pixel of a satellite image to estimate biomass and subsurface light fields, and these combined with a light algorithm to model primary production for the Benguela large marine ecosystem.

Phytoplankton class‐specific primary production in the world's oceans: Seasonal and interannual variability from satellite observations

We apply an innovative approach to time series data of surface chlorophyll from satellite observations with SeaWiFS (Sea‐viewing Wide Field‐of‐view Sensor) to estimate the primary production associated with three major phytoplankton classes (micro‐, nano‐, and picophytoplankton) within the world's oceans. Statistical relationships, determined from an extensive in situ database of phytoplankton pigments, are used to infer class‐specific vertical profiles of chlorophyll a concentration from satellite‐derived surface chlorophyll a. This information is combined with a primary production model and class‐specific photophysiological parameters to compute global seasonal fields of class‐specific primary production over a 10‐year period from January 1998 through December 2007. Microphytoplankton (mostly diatoms) appear as a major contributor to total primary production in coastal upwelling systems (70%) and temperate and subpolar regions (50%) during the spring‐summer season. The contribution of picophytoplankton (e.g., prokaryotes) reaches maximum values (45%) in subtropical oligotrophic gyres. Nanophytoplankton (e.g., prymnesiophytes) provide a ubiquitous, substantial contribution (30–60%). Annual global estimates of class‐specific primary production amount to 15 Gt C yr−1 (32% of total), 20 Gt C yr−1 (44%) and 11 Gt C yr−1 (24%) for micro‐, nano‐, and picophytoplankton, respectively. The analysis of interannual variations revealed large anomalies in class‐specific primary production as compared to the 10‐year mean cycle in both the productive North Atlantic basin and the more stable equatorial Pacific upwelling. Microphytoplankton show the largest range of variability of the three phytoplankton classes on seasonal and interannual time scales. Our results contribute to an understanding and quantification of carbon cycle in the ocean.

Flow effects on benthic grazing on phytoplankton by a Caribbean reef

We present measurements of flows and fluxes of phytoplankton to Conch Reef, Florida, a Caribbean reef dominated by sponges and soft corals, located in 15 m of water offshore of Key Largo. Vertical profiles of chlorophyll a, a proxy for phytoplankton biomass, showed a near‐bed depletion, indicating the existence of concentration boundary layers. Along with simultaneous measurements of velocity profiles, near‐bed turbulence, and temperature stratification, these profiles were used to compute α, the mass transfer velocity of phytoplankton to the bed (i.e., the flux to the bed normalized by near‐bed concentration). The α value ranged from ‐40 to +130 m d−1, with a significant linear positive relationship with shear velocity. The median value of α = 48 ± 20 m d−1 is larger than would be expected, given the observed population of filter‐feeding sponges. Nonetheless, these large values of a are consistent with values found recently for another coral reef as well as for a soft bottom estuarine community. Taken as a whole, these measurements indicate that reefs with large roughness and/or energetic currents should be able to support higher biomasses of benthic organisms than would low relief reefs or reefs in sluggish waters.

The effect of extreme rainfall on summer succession and vertical distribution of phytoplankton in a lacustrine part of a eutrophic reservoir

During the summer of 2006, phytoplankton succession and vertical distribution were studied in the eutrophic Řimov Reservoir (Czech Republic). Disturbances in the form of dramatic summer storms had a crucial effect on phytoplankton succession and its vertical distribution. Two extreme rainfalls substantially increased flushing rate, yielding a fairly short retention time within the reservoir that disrupted established thermal stratification. The first storm initiated the development of summer phytoplankton, while the second storm reversed the phytoplankton succession to an earlier stage. The measurement of vertical profiles of chlorophyll a showed that the phytoplankton were heterogeneously distributed in the water column and formed subsurface diatom maxima (SDM) over most of the summer. Biogenic silica deposition studied by PDMPO (2-(4-pyridyl)-5{[4-dimethylaminoethyl-aminocarbamyl)-methoxy] phenyl}oxazole) was measured to compare diatom growth rates over the season and between surface and depth of SDM. Diatoms deposited 20 times more silica at the surface than at the SDM depth, although only a half of diatom population was involved in the silica deposition. Maximum rates of silica deposition were measured in samples just after both summer storms events. When diatoms dominated the phytoplankton, the amounts of available Si and P in water were significantly correlated (r2 = 0.81, p < 0.001, df = 42). Outliers from the linear relationship were found only during a period when substantial picocyanobacterial population developed in the reservoir. Due to their lack of Si requirements, ambient Si concentration markedly increased while P was depleted to growth limiting levels. Temporal variations in Si:P ratio therefore illustrate the importance of resource competition in phytoplankton seasonal succession.

Chlorophyll content of Plešné Lake phytoplankton cells studied with image analysis

Using image analysis, chlorophyll autofluorescence was measured in single cells of green alga Monoraphidium dybowskii and in filaments of cyanobacteria (Pseudanabaena sp. and Limnothrix sp.) in the vertical profile of small acidified mountain lake Plesne jezero (Plesne Lake) from May to November of 2003. Cell chlorophyll autofluorescence was converted to cell chlorophyll content using a conversion factor determined by comparing the total autofluorescence of phytoplankton in a microscope field with spectrophotometrically determined total chlorophyll concentration; the conversion factor did not differ between epilimnion (0.5 m depth) and hypolimnion (9 m depth). Vertical patterns of chlorophyll concentration and of cellular chlorophyll content depended on water column mixing: during the period of stable thermal stratification, a metalimnetic peak in total chlorophyll concentration was present and cellular chlorophyll contents in the metalimnion and hypolimnion were notably elevated compared to the surface. Monotonous vertical profiles of both total chlorophyll concentration and cell chlorophyll content were typical for the period of water column overturn. During the stratification period, hypolimnetic Monoraphidium cell chlorophyll content was on average twice as high (maximum difference 2.7-fold) compared to surface values (of 3.2–12.9 fg µm−3), while in filamentous cyanobacteria (surface cell chlorophyll content of 2.2–13.3 fg µm−3), the difference was much higher — six-fold on average, with an 11.6-fold maximum value. The values measured with image analysis in 2003 were compared to unpublished values of total phytoplankton biomass-specific chlorophyll concentrations obtained using manual phytoplankton biomass determination and spectrophotometric chlorophyll measurement in 1998 at the same locality. Good agreement was found in seasonal patterns and vertical profiles of chlorophyll between both seasons.

Phytoplankton distribution in the Bismarck Sea north of Papua New Guinea: The effect of the Sepik River outflow

Chlorophyll and carotenoid pigments were measured at stations in the Bismarck Sea north of Papua New Guinea and along the equator between 143°E and 152°E as part of the Tropical River Ocean Processes In Coastal Settings (TROPICS) programme in August 1997 and January/February 2000. Algal class abundances were estimated from these pigments using the CHEMTAX programme. The algal classes likely to be present (based on microscopy, HPLC chromatograms, literature and unpublished data from the region) and estimated by CHEMTAX were diatoms, cryptophytes, chlorophytes, haptophytes, chrysophytes, Synechococcus and Prochlorococcus. The first period corresponded to the SE tradewind season, when there was a drought in Papua New Guinea and the discharge from the Sepik River was unusually low. Samples were collected on four N–S transects (approximately 150 km in length): one to the east of the Sepik River, one from the mouth of the Sepik River and two to the west of the Sepik River. Stations in the central Bismarck Sea and along the equator were characterised by a deep chlorophyll maximum (DCM), and haptophytes and Prochlorococcus were the major algal classes. Stations affected by the Sepik River plume were characterised by a more homogeneous distribution of chlorophyll with chlorophytes making a significant contribution, particularly at shallower depths. Synechococcus and chrysophytes were moderately abundant, while cryptophytes and diatoms were of minor importance. The second cruise occurred during the NW monsoon under conditions of high river flow, and samples were collected along a transect from the mouth of the Sepik River to Manus Island and in the central Bismarck Sea. Chlorophyll concentrations were higher than in 1997, particularly in near surface waters, while a DCM was generally not observed at any station in the Bismarck Sea—even at stations upstream (and upwind) and far removed from the mouth of the Sepik River. Prochlorococcus made a smaller, and chlorophytes a larger, contribution to chlorophyll than in 1997. Unlike the situation in 1997, there were large station to station variations in the vertical profiles of chlorophyll and algal class distributions which cannot be attributed directly to the plume from the Sepik River. Pigment ratios, fitted by the CHEMTAX programme, were generally in good agreement with literature values.

Rapid reworking of subtidal sediments by burrowing spatangoid urchins

The mixing and displacement of sediment by benthic macrofauna (bioturbation) has major biogeochemical implications, and can control rates of organic matter degradation and carbon burial. Large, abundant, mobile macrofauna often dominate sediment bioturbation, and heart urchins of the genus Echinocardium are regarded as key sediment bioturbators in marine systems throughout the world. To better understand the bioturbation potential and functional role of Echinocardium, we developed a mathematical model and parameterized it with field data from six locations in northern New Zealand in order to estimate bioturbation rates in these places. Although urchin sizes and densities were measured in consecutive years at all six locations, we obtained a third model parameter, urchin movement rate, from one time and place only (Site OB5). Because confidence in model output was greatest at OB5, and since OB5 had the highest sediment reworking rate of all sites, our model yielded a good upper bound estimate for the bioturbation potential of Echinocardium in the areas examined. The volume of sediment displaced by Echinocardium populations reached 20,000 cm 3 m −2 d −1 at OB5, suggesting that surface sediment is reworked about every 3 days at sites where Echinocardium is abundant. Experimental work with a fluorescent tracer at OB5 suggested limited downward particle movement as a result of Echinocardium bioturbation, though vertical profiles of chlorophyll a and organic matter content indicated well mixed sediment. The loss of Echinocardium because of broad-scale anthropogenic disturbance to the seabed could have major consequences on marine ecosystem functioning.