Estimating Phytoplankton Biomass Research Articles

Influence of mini warm pool extent on phytoplankton productivity and export in the Arabian sea

The Arabian Sea is a highly productive tropical ecosystem of the Indian Ocean that supports high fluxes of particulate organic carbon to the mesopelagic zone from two distinct periods of elevated biological productivity associated with the semiannual reversals of the monsoonal wind system. There are now strong indications that the Arabian Sea's monsoonal wind patterns and hydrographic conditions are being impacted by long-term temperature increases, but the consequences of these changes on primary production and carbon export to the mesopelagic zone are unknown. This is especially true for the summer monsoon period when cloud cover obscures much of the Arabian Sea basin and therefore precludes remotely sensed ocean color measurements for estimating phytoplankton biomass and productivity. Here we overcome this limitation by using a database of bio-optical profiles from Biogeochemical Argo floats collected over the last decade to evaluate the impact of interannual temperature increases on Arabian Sea primary production and carbon export. We classify individual years of float observations based on the spatial extent of the Arabian Sea Mini Warm Pool that appears in the southeast Arabian Sea before the onset of the summer monsoon. This Mini Warm Pool, which begins to build in winter and collapses with the onset of the summer monsoon in late spring, has gained considerable interest on account of its influence on the timing of the onset of the summer monsoon. We observed a 35 percent decrease in primary production during the summer monsoon phytoplankton bloom in strong warm pool years, and a 13 percent decrease in particle stocks in the upper mesopelagic zone following the peak of the bloom. Decreases in production and export were additionally accompanied by a decrease in average particle size, indicating a shift from larger cells like diatoms that appear from fertilization of the oligotrophic waters to smaller phytoplankton size classes in response to a deepening of the thermocline and increased stratification of the water column. These results suggest changes in phytoplankton community structure and further decreases in primary production and carbon export in the Arabian Sea in response to future warming.

Deriving vertical profiles of chlorophyll-a concentration in the upper layer of seawaters using ICESat-2 photon-counting lidar.

Chlorophyll-a concentration (chl-a) is a great indicator for estimating phytoplankton biomass and productivity levels and is also particularly useful for monitoring the water quality, biodiversity and species distribution, and harmful algal blooms. A great deal of studies investigated to estimate chl-a concentrations using ocean color remotely sensed data. With the development of photon-counting sensors, spaceborne photon-counting lidar can compensate for the shortcomings of passive optical remote sensing by enabling ocean vertical profiling in low-light conditions (e.g., at night). Using geolocated photons captured by the first spaceborne photon-counting lidar borne on ICESat-2 (Ice, Cloud, and Land Elevation Satellite-2), this research reported methods for deriving vertical profiles of chl-a concentration in the upper layer of ocean waters. This study first calculates the average numbers of backscattered subaqueous photons of ICESat-2 at different water depths, and then estimates the optical parameters in water column based on a discrete theoretical model of the expected number of received signal photons. With the estimated optical parameters, vertical profiles of chl-a concentration are calculated by two different empirical algorithms. In two study areas (mostly with Type I open ocean waters and small part of Type II coastal ocean waters), the derived chl-a concentrations are generally consistent when validated by BGC-Argo (Biogeochemical Argo) data in the vertical direction (MAPEs<15%) and compared with MODIS (Moderate Resolution Imaging Spectroradiometer) data in the along-track direction (average R2>0.86). Using globally covered ICESat-2 data, this approach can be used to obtain vertical profiles of chl-a concentration and optical parameters at a larger scale, which will be helpful to analyze impact factors of climate change and human activities on subsurface phytoplankton species and their growth state.

Improving the Retrieval of Carbon-Based Phytoplankton Biomass from Satellite Ocean Colour Observations

Phytoplankton is at the base of the marine food web and plays a fundamental role in the global carbon cycle. Ongoing climate change significantly impacts phytoplankton distribution in the ocean. Monitoring phytoplankton is crucial for a full understanding of changes in the marine ecosystem. To observe phytoplankton from space, chlorophyll-a concentration (Chl) has been widely used as a proxy of algal biomass, although it can be impacted by physiology. Therefore, there has been an increasing focus towards estimating phytoplankton biomass in units of carbon (Cphyto). Here, we developed an algorithm to quantify Cphyto from space-based observations that accounts for the spatio-temporal variations of the backscattering coefficient associated with the fraction of detrital particles that do not covary with Chl. The main findings are: (i) a spatial and temporal variation of the detritus component must be accounted for in the Cphyto algorithm; (ii) the refined Cphyto algorithm performs better (relative bias of 23.7%) than any previously existing model; and (iii) our algorithm shows the lowest error in Cphyto across areas where picophytoplankton dominates (relative bias of 14%). In other areas, it is currently not possible to accurately assess the performance of the refined algorithm due to the paucity of in situ carbon data associated with nano- and micro-phytoplankton size classes.

Fluvial influence on the biochemical composition of particulate organic matter in the Laptev and Western East Siberian seas during 2015

Here, we investigated the elemental (C/N ratio) and isotopic signatures (δ13C) and major biomolecules (carbohydrates, proteins, and lipids) and their relative abundance (i.e., the biochemical composition) in particulate organic matter (POM) to assess their origin and fate in the Laptev and western East Siberian seas during late summer/fall of 2015. In addition, we compared our results with the summer data of 2013 collected from Laptev and northwestern East Siberian seas. In accordance with the observed hydrological structure (i.e., a northward, warmer, diluted freshwater plume than previously observed in 2013), the more depleted δ13C (−28.2 ± 0.9‰) and higher C/N ratio (10.8 ± 2.0) than those of 2013 signalled that fluvially released terrestrial organic carbon (TerrOC) was the main source of the POM, unlike in 2013, when phytoplankton was the dominant source (δ13C = −24.9 ± 1.0‰, C/N ratio = 7.6 ± 2.4; Ahn et al., 2019). During the offshore transport of heterogeneous TerrOC, carbohydrates seem to be the primary contributor to the bulk POM as a result of selective degradation and hydrodynamic sorting. Despite the TerrOC-dominated system in 2015, some marine influence was also found. The estimated phytoplankton biomass was low and comparable among the study sites. In addition, the presence of resting spores and high ammonium concentrations within the water column may suggest senescent and, to some extent, degrading conditions of the resident phytoplankton. In this regard, carbohydrate concentrations and freshwater content were significantly correlated (r = 0.79, p < 0.01), suggesting that carbohydrates are useful inferences of freshwater within overall study sites, at least when the marine influence is similar or low.

Correcting in situ chlorophyll fluorescence time-series observations for nonphotochemical quenching and tidal variability reveals nonconservative phytoplankton variability in coastal waters.

Chlorophyll fluorometry is one of the most commonly implemented approaches for estimating phytoplankton biomass in situ, despite documented sources of natural variability and instrumental uncertainty in the relationship between in vivo fluorescence and chlorophyll concentration. A number of strategies are employed to minimize errors and quantify natural variability in this relationship in the open ocean. However, the assumptions underlying these approaches are unsupported in coastal waters due to the short temporal and small spatial scales of variability, as well as the optical complexity. The largest source of variability in the in situ chlorophyll fluorometric signal is nonphotochemical quenching (NPQ). Typically, unquenched nighttime observations are interpolated over the quenched daytime interval, but this assumes a spatial homogeneity not found in tidally impacted coastal waters. Here, we present a model that provides a tidally resolved correction for NPQ in moored chlorophyll fluorescence measurements. The output of the model is a time series of unquenched chlorophyll fluorescence in tidal endmembers (high and low tide extremes), and thus a time series of phytoplankton biomass growth and loss in these endmember populations. Comparison between modeled and measured unquenched time series yields quantification of nonconservative variations in phytoplankton biomass. Tidally modeled interpolation between these endmember time series yields a highly resolved time series of unquenched daytime chlorophyll fluorescence values at the location of the moored sensor. Such data sets provide a critical opportunity for validating the satellite remotely sensed ocean color chlorophyll concentration data product in coastal waters.

Recommendations for obtaining unbiased chlorophyll estimates from in situ chlorophyll fluorometers: A global analysis of WET Labs ECO sensors

Chlorophyll fluorometers provide the largest in situ global data set for estimating phytoplankton biomass because of their ease of use, size, power consumption, and relatively low price. While in situ chlorophyll a (Chl) fluorescence is proxy for Chl a concentration, and hence phytoplankton biomass, there exist large natural variations in the relationship between in situ fluorescence and extracted Chl a concentration. Despite this large natural variability, we present here a global validation data set for the WET Labs Environmental Characterization Optics (ECO) series chlorophyll fluorometers that suggests a factor of 2 overestimation in the factory calibrated Chl a estimates for this specific manufacturer and series of sensors. We base these results on paired High Pressure Liquid Chromatography (HPLC) and in situ fluorescence match ups for which non-photochemically quenched fluorescence observations were removed. Additionally, we examined matchups between the factory-calibrated in situ fluorescence and estimates of chlorophyll concentration determined from in situ radiometry, absorption line height, NASA's standard ocean color algorithm as well as laboratory calibrations with phytoplankton monocultures spanning diverse species that support the factor of 2 bias. We therefore recommend the factor of 2 global bias correction be applied for the WET Labs ECO sensors, at the user level, to improve the global accuracy of chlorophyll concentration estimates and products derived from them. We recommend that other fluorometer makes and models should likewise undergo global analyses to identify potential bias in factory calibration.

Cálculo del contenido celular de carbono de cuatro taxones de diatomeas: biovolumen y espectrofotometría

Background. Phytoplankton biomass is an essential parameter in aquatic ecosystems; two of the most widely used indicators for estimating phytoplankton biomass are chlorophyll-a (Chl-a) by spectrophotometry and biovolume (BV). Goals. To show how different results are when estimating phytoplankton biomass by means of the spectrophotometric and BV methods. Methods. Biomass by Chl-a and BV were estimated in four cultures of marine diatoms. For estimating BV, we used live cells and acid Lugol’s solution fixed cells. Results. Biomass values were noticeably different: the BV for Thalassiosira hispida and Skeletonema costatum underestimates ≈41% of the biomass with respect to values obtained by spectrophotometry. In contrast, the BV for Pseudo-nitzschia sp. and Cylindrotheca closterium overestimates the biomass by 4% and 25%, respectively. In the case of estimated biovolume, the results indicate that fixer acid Lugol’s solution modified the size of the cell significantly in C. closterium and Pseudo-nitzschia sp. (p 0.05). Two equations are provided for biomass calculation when studying live (pgC/cell = -1.5567 + 0.1428 (BV)) or acid Lugol’s solution fixed (pgC/cell = -5.0126 + 0.1644 (BV)) diatom cells. Conclusions. Although BV is useful, it can have drawbacks because it produces values above and below those estimated by spectrophotometry, in addition to the fact that acid Lugol´s solution modifies cell size. We would advise considering the available information based in Chl-a or carbon of different species.

Lotic bacterioplankton and phytoplankton community changes under dissolved organic-carbon amendment: evidence for competition for nutrients

During periods of low river discharge, bacterial growth is typically limited by dissolved organic carbon (DOC) and is tightly regulated by phytoplankton production. However, import of allochthonous DOC into rivers by freshwater inflows may diminish bacterial reliance on phytoplankton-produced carbon, leading to competition for nitrogen (N) and phosphorus (P). To investigate phytoplankton–bacterial competition in response to allochthonous inputs, we conducted a mesocosm experiment, comparing microbial responses to the following two manipulation treatments: (1) addition of N and P, and (2) addition of a DOC and N and P. Measurement of chlorophyll-a estimated phytoplankton biomass and microscopic counts were performed to discriminate community change. Bacterial abundance was tracked using flow cytometry and community assemblages were characterised using automated ribosomal intergenic spacer analyses and 16S rRNA-amplicon sequencing. We found that bacterial abundance increased in the leachate addition, whereas chlorophyll-a was reduced and the bacterial community shifted to one dominated by heterotrophic genera, and autotrophic microbes including Synechococcus and Cyclotella increased significantly in the nutrient treatment. These observations indicated that DOC and nutrient inputs can lead to shifts in the competitive dynamics between bacteria and phytoplankton, reducing phytoplankton biomass, which may potentially shift the major pathway of carbon to higher trophic organisms, from the phytoplankton grazer chain to the microbial food web.

광합성 색소의 HPLC 분석을 위한 여과지 분쇄 효과 평가

High-Performance Liquid Chromatography (HPLC) is a widely used method for measuring the concentration of chlorophyll a as an indicator for estimating phytoplankton biomass and primary production and also for identifying carotenoids to determine phytoplankton composition. However, tissue grinding procedure requires a lot of time and experience in the analysis of multiple sample. Accordingly, we measured the concentrations of photosynthetic pigments before and after the grinding, in order to understand the grinding effects on the quantitative analysis of chlorophylls and carotenoids using samples from southwestern East Sea. When tissue grinding procedure was omitted, we found that Chl a concentrations were underestimated up to 45% in average. Also, concentrations of Zeaxanthin, 19`-butanoyloxyfucoxanthin, 19`-hexanoyloxyfucoxanthin, biomarkers of pico and nano-size phytoplankton, were underestimated up to maximum 77~85% without grinding. We found that the smaller the phytoplankton, the bigger underestimation of their biomarker pigments concentration is likely to happen due to the incomplete extraction. Thus, tissue grinding procedure should be included for HPLC analysis in all cases, to prevent the underestimation of not only Chl a but also carotenoids pigments.

Phytoplankton Community Shifts and Harmful Algae Presence in a Diversion Influenced Estuary

River water entering estuaries affects the physical and chemical environment at irregular intervals creating a highly dynamic aquatic habitat. Phytoplankton are important primary producers in estuaries that respond quickly to their changing environment. Over a 12-month period, the phytoplankton response was examined in terms of biomass, abundance, community composition, and potential phycotoxin production to seasonal changes in river input entering the Breton Sound Estuary in Louisiana. Chlorophyll a (chl a) measurements estimated phytoplankton biomass and light microscopy identified phytoplankton abundance and community composition. Phycotoxins were measured using ultra-sensitive enzyme-linked immunosorbent assay (ELISA). During the study period, chl a and cell abundances exhibited an inverse relationship with the seasons, which were determined by temperature and diversion input rates. The community was dominated by cyanobacteria for most of the year, mainly corresponding to the warm-low flow season. For the rest of the year, cyanobacteria decreased while chlorophytes and centric diatoms increased to approximately equal contributions. Overall, the phytoplankton community composition was most commonly moderated by temperature, salinity, and dissolved inorganic nitrogen (DIN) availability using BIO-ENV. Microcystins (MCs) were detected throughout the year ranging from below detection to 2.92 μg MCs l−1. MC levels were highest during the warm-low flow season and toward the lower estuary. The detection of MCs for the first time in the Breton Sound Estuary illustrates a potential risk to human health and economically important estuarine food webs.

Performance of absorption coefficient measurements for the in situ determination of chlorophyll-a and total suspended matter

The concentrations of chlorophyll-a ([chl-a]) and total suspended matter ([TSM]) are important parameters in biological oceanography. [Chl-a] is a commonly used proxy for estimating phytoplankton biomass while [TSM] also includes detrital material and mineral particles and thus influences light attenuation and photosynthetic activity in the water column.For characterizing the distribution (patchiness) of both parameters adequately over a longer time period, fast and effective measurement methods are required that can also be applied in situ or continuously. Thus, alternatively to direct determination of [chl-a] and [TSM], optical proxy values are often measured. The PSICAM is an integrating cavity approach for measuring absorption coefficients of water constituents with high precision which can be used also continuously (flow-through-PSICAM). In this study, the performance of these absorption measurements for [chl-a] and [TSM] determination was evaluated and compared with the performance of traditional approaches using chl-a fluorescence and turbidity measurements.Data were collected in the German Bight (North Sea) in 2010 and 2011. For [chl-a], fluorescence measurements are compared with pigment absorption coefficient values at a wavelength of 676nm (aΦ 676nm), while the [TSM]-proxies were turbidity and particle absorption at 700nm (ap 700nm). As reference data, HPLC-determined [chl-a] and gravimetrically determined [TSM] were used.Our results showed linear relationships between [chl-a] and fluorescence or aΦ 676nm, respectively. Coefficients of determination (R2) were in a range of 0.71 to 0.88, with the higher values related to the absorption measurements. Furthermore, it was demonstrated that fluorescence underestimates [chl-a] depending on ambient photosynthetically active radiation (PAR). Linear relationships were also observed between [TSM] and its optical proxies with R2 values between 0.93 and 0.98. Turbidity measurements appeared to be influenced to a certain extent by the physical properties of the suspended material, resulting in a slightly higher variability than the ap 700nm measurements.Absorption measurements turned out to be promising optical proxies for determining [TSM] and [chl-a] due to their lower variability compared with the other proxies. This improved accuracy could be already partially achieved also for continuous measurements. Moreover, a combination of the different optical methods has the potential to provide additional information besides concentration, such as the source of TSM in the water or physiological condition of the phytoplankton.

Managing wastewater effluent to enhance aquatic receiving ecosystem productivity: A coastal lagoon in Western Australia

Large amounts of waste are generated in urban centers that if properly managed could promote ecological services. In order to promote nutrient cycling and productivity without endangering aquatic ecosystems, management of wastewater treatment and effluent discharges to receiving waters must be assessed on a case-by-case basis. We applied this premise to examine a municipal wastewater treated effluent discharge in a shallow oligotrophic coastal lagoon in Western Australia. Three-dimensional hydrodynamic-ecological modeling (ELCOM-CAEDYM) was used to assess the reaction of ecosystem for effluent quality. Two scenarios were evaluated for the summer 2000-2001 period, the actual or "current" (conventional secondary treatment) and an "alternative" (involving substitution of biological nutrient removal by advanced treatment). The residence time of the simulated numerical domain averaged 8.4 ± 1.3 days. For the current scenario the model successfully estimated phytoplankton biomass, as chlorophyll-a concentration (Chl-a), that is within field-measured ranges and previously recorded levels. The model was able to reproduce nitrogen as the main limiting nutrient for primary production in the coastal ecosystem. Simulated surface Chl-a means were 0.26 (range 0.19-0.38) μg Chl-a/L for the current scenario and 0.37 (range 0.19-0.67) μg Chl-a/L for the alternative one. Comparison of the alternative scenario with field-measured Chl-a levels suggests moderate primary production increase (16-42%), within local historical variability. These results, suggest that such a scenario could be used, as part of a comprehensive wastewater management optimization strategy, to foster receiving ecosystem's productivity and related ecological services maintaining its oligotrophic state.

Nitrogen nutrition in assemblages dominated by Pseudo-nitzschia spp., Alexandrium catenella and Dinophysis acuminata off the west coast of South Africa

A study was carried out in the southern Benguela upwelling system in 2006 and 2007 to establish how the nutrient regime determines community succession and the selection of harmful algal bloom species. In March 2006, Pseudo-nitzschia spp. reached concentrations of 13 × 10 6 cells l -1 , representing 80% of the total estimated phytoplankton biomass, while chlorophyll a (chl a) reached 57 µg l -1 . High rates of NO3 - uptake (ρNO3 -, maximum 0.56 µmol N l -1 h -1 ) led to NO3 - depletion (<0.1 µM). However, cell numbers remained high for several days, sustained by regenerated nitro- gen, with ƒ-ratios dropping from 0.79 to 0.12. In March 2007, a bloom of Alexandrium catenella (4.5 × 10 5 cells l -1 , 77% of the phytoplankton biomass) occurred, with surface chl a concentrations of 26 µg l -1 . NO3 - concentrations were high (10 to 17 µM), sustaining high ρNO3 - (maximum 0.61 µmol N l -1 h -1 ) and ƒ-ratios up to 0.87. In April, Dinophysis acuminata reached 3.1 × 10 4 cells l -1 (91% phytoplank- ton biomass), with NO3 - concentrations <0.5 µM and ƒ-ratios <0.1, indicative of regenerated produc- tion. Nutrient uptake kinetics showed that Pseudo-nitzschia spp. displayed the highest maximum specific uptake rates (15.0 × 10 -3 and 18.0 × 10 -3 h -1 for NO3 - and NH4 + , respectively). D. acuminata displayed the highest affinity for NH4 + , as shown by its higher α (slope of the nutrient uptake vs. con- centration curve) of 20.7 × 10 -3 , compared to 13.4 × 10 -3 and 5.9 × 10 -3 h -1 (µmol N l -1 ) -1 for Pseudo- nitzschia spp. and A. catenella, respectively. D. acuminata was an affinity strategist, successful in nitrogen-depleted waters, whereas both Pseudo-nitzschia spp. and A. catenella were velocity strate- gists, better adapted to utilising high NO3 - concentrations during upwelling pulses.

Estimating phytoplankton biomass in coastal waters of Alaska using airborne remote sensing

Empirical airborne remote-sensing relationships were examined to estimate chlorophyll a concentration in the first optical depth (chl FOD) of coastal waters of Afgonak/Kodiak Islands during July–August 2002. Band-ratio and spectral-curvature models were tested using satellite remote-sensing reflectance ( R rs( λ)) measurements. Additional shipboard and airborne R rs( λ) data were also analysed to evaluate consistency of proposed chl FOD– R rs( λ) relationships. Validation of chlorophyll algorithms was performed using data collected in the northern-part of the Gulf of Alaska and Bering Sea during 1996, 2002, and 2003 cruises. Likewise, oceanographic conditions during the surveys were typified to interpret variability of chl FOD fields. The SeaWiFS band-ratio algorithm OC2d was the most sensitive R rs combination ( R rs(509)/ R rs(553)) to detect chl FOD variability. Conversely, OC2a ( R rs(412)/ R rs(553)) had the lowest performance to derive chl FOD values. No valid statistical regressions were established for spectral–curvature relationships in the blue spectrum (< 500 nm). Fertile waters (> 5 mg m − 3 ) were preferentially located over shallow banks (∼50 m) and at the entrance of the bays. The approach used in this study to derive chl FOD values could be universal for Alaskan coastal waters. However, chl FOD– R rs( λ) relationships must be calibrated locally for a given season.

Phytoplankton community growth-rate response to nutrient pulses in a shallow turbid estuary, Galveston Bay, Texas

Phytoplankton growth is a physiological process often limited by temperature, nutrients or light, while biomass accumulation is a function of growth rates, grazing and deposition. Although primary productivity measurements are usually used to assess responses to limiting factors, the rates are proportional to biomass and inversely related to grazing pressure during experimental incubations. Alternatively, carbon-specific growth-rate determinations provide insights into physiological responses without the confounding acts of biomass and grazing. The objective of this study was to quantify the growth-rate responses of phytoplankton to enhanced nutrient availability (nitrate and phosphate) over a range of in situ irradiances. Growth rates were determined based on chlorophyll a-specific 14 C-uptake rates by phytoplankton. Phytoplankton demonstrated high (24 h) growth rates when exposed to increased concentrations of limiting nutrients, independent of the surface irradiances (12-41%). Growth-rate responses were also compared zenith the biomass (chlorophyll a) responses and community composition. Observed and estimated phytoplankton biomass changes during the incubations differed, emphasizing the structural role of graders on the phytoplankton community. The phytoplankton community in Galveston Bay has the potential to instantaneously respond to nutrient pulses, facilitating diatom biomass accumulations in spring and summer and small, flagellated species and cyanobacteria during periods of low nutrient inputs. Thus, Galveston Bay phytoplankton biomass and community composition reflect a dynamic balance between the frequency of nutrient pulsing and grazing intensity.

Latitudinal comparisons of equatorial Pacific zooplankton

Abstract Zooplankton biomass and rates of ingestion, egestion and production in the equatorial Pacific Ocean along 140°W and 180° exhibit maximum values in the High-Nutrient Low-Chlorophyll (HNLC) zone associated with equatorial upwelling (5°S–5°N) as compared to the more oligotrophic regions to the north and south. Zooplankton biomass and rates are not usually highest on the equator, but increase “downstream” of the upwelling center as the zooplankton populations exhibit a delayed response to enhanced phytoplankton production. The vertical distribution of zooplankton biomass in the equatorial HNLC area tends to be concentrated in surface waters and is more uniform with depth in oligotrophic regions to the north and south of the equatorial upwelling zone. In general, the amount of mesozooplankton (>200 μm) carbon biomass is approximately 25% of estimated phytoplankton biomass and 30% of bacterial biomass in the HNLC area of the equatorial Pacific Ocean. Zooplankton grazing on phytoplankton is low in the equatorial Pacific Ocean, generally −1 for 64–200 μm zooplankton to 0.08 d −1 for 1000–2000 μm zooplankton. Thus, the numerical and functional response of equatorial zooplankton to increases in phytoplankton production are more rapid than normally occurs in sub-tropical and temperate waters. Potential zooplankton fecal pellet production, estimated from metabolic demand, is approximately 1.6 times the estimated gravitational carbon flux at 150 m in the zone of equatorial upwelling (5°S–5°N) and 1.1 times the export flux in the more oligotrophic regions to the north and south. The active flux of carbon by diel migrant zooplankton in the HNLC zone is a minor fraction of the gravitational flux (2% at 140°W, 4% at 180°) but increases in the more oligotrophic regions to the north and south where there is a deeper mixed layer and a greater relative proportion of diel migrant zooplankton.

Phytoplankton biomass, production and grazing mortality in Exmouth Gulf, a shallow embayment on the arid, tropical coast of Western Australia

Phytoplankton biomass, production and grazing mortality were measured in Exmouth Gulf, a shallow embayment on the arid, tropical coast of Western Australia. In the Gulf, chlorophyll a concentrations were typically 0.2–0.3 mg m −3 and phytoplankton production rates were mostly below 25 mg of C m −3 d −1. The low phytoplankton biomass and production in the Gulf are seemingly related to the aridity of the region and hence the small terrestrial runoff of nutrients. In dilution experiments, which complemented 14C incorporation experiments, the proportion of potential primary production grazed ranged from 79 to 155% in the fluorometric analysis of chlorophyll a and from 73 to 191% in the HPLC analysis of chlorophyll a. There may be some excess phytoplankton production on the relatively well flushed, western side of the Gulf. On the eastern side of the Gulf, however, the estimated grazer biomass ranged between 4.6–8.8 mg of C m −3, not much less than the estimated phytoplankton biomass (6–15 mg of C m −3), and this grazer population appeared to consume more organic matter than the phytoplankton population could produce. The disproportionally large grazer biomass and the relatively high grazing mortality of phytoplankton may be due to the supply of additional organic matter from benthic macroalgal communities and/or mangrove and salt flat systems present on the eastern side of the Gulf.

Copepod gut contents, ingestion rates and grazing impact on phytoplankton in relation to size structure of zooplankton and phytoplankton during a spring bloom

Copepod gut contents, ingestion rates a s well as the grazing impact on phytoplankton were estimated in coastal waters off Plymouth (SW England) before and during the spring phytoplankton bloom in 1989. A size-fractionation approach was applied, estimating phytoplankton biomass as chlorophyll a concentration in 3 size fractions [pico(0.2 to 2 pm), nano(2 to 10 pm) and microplankton (10 to 100 pm)] and copepod abundances, gut pigment contents and gut evacuation rates in 3 size categories (small, 200 to 350 pm; medium, 350 to 710 pm; and large. > 710 pm). Results showed a direct relation between copepod feeding and the size structure of zooplankton and phytoplankton. Gut pigment contents were higher for the larger copepods and lower for the smaller ones, and increased in parallel with the increase of chlorophyll a concentration and dominance of large cells (> 10 pm) during the spring bloom. Estimations of the grazing impact of the copepod community showed that 5 to 8 % of the total phytoplankton standing stock, and up to between 30 and 40 % of the fraction > 10 Ltm, was grazed during the exponential growth phase of phytoplankton prior to the spring bloom, in contrast to < 2 % and < 5 %, respectively, during other periods before and after the bloom. The medium-sized copepods were responsible for most of this grazing impact, a s a result of their numerical dominance in these coastal waters. Different aspects affecting these estimations of copepod community grazing impact are discussed.

Allometric estimation of the productivity of phytoplankton assemblages

An allometric model is used to estimate the productivity of natural phytoplankton assemblages. The 14C fixation rates of > 5 ~m phytoplankton cells, measured over a 5 yr period in the Celtic Sea, are used to test the relationship between cell size and productivity of natural phytoplankton assemblages. Phytoplankton biomass, as estimated from cell volume, explains between 85 and 90 % of the variance in 14C fixation rate under optimum growth conditions; in comparison, chlorophyll a concentration explains only 59 % of the variance. An allometric model, applied to the estimated phytoplankton biomass, is used to calculate the potential productivity of the phytoplankton under optimum growth conditions; this estimate of phytoplankton productivity explains 93 % of the variance in the in situ 14C fixation rate. When applied to data obtained over a 4 d period in the Celtic Sea, differences are found in the growth rate of individual species as well as in presumed losses due to grazing and sedimentation. This approach has the potential to estimate the productivity of each of the individual phytoplankton species which comprise a natural assemblage.