Size-fractionated Chlorophyll-a Research Articles

Function rather than structure of phytoplankton community reveals changes of water quality in an ecological restored lake.

Although phytoplankton is well known as robust bioindicators to aquatic environments, their indicating functions based on different community parameters remain to be understood. In order to filter effective bioindicators in aquatic ecosystems, four phytoplankton community parameters including species richness (SR), total biomass (SBP), functional groups (FGBP), and size-fractionated chlorophyll-a (SC) were demonstrated in a subtropical artificial lake with ecological restoration in South China. Our results indicated that all the above four parameters exhibited high sensitivity to environmental variations and illustrated distinct aspects of indicating functions to aquatic environments due to their individual biological characteristics. Based on FGBP, both spatial and temporal differences in phytoplankton community could be identified. SR and SBP only classified the spatial and temporal distributions, respectively, while SC could distinguish the sewage outfalls from other sites. In terms of ecological management, two parameters (SR and FGBP) could distinguish the restored waters from untreated environments as non-point source pollution, and another parameter SC could indicate the sewage outfalls as point source pollution. Therefore, the combination of the above two categories of phytoplankton community parameters could make the strongest indicating functions. Our study provided greater insight into indicating functions of phytoplankton community parameters in an ecological restored lake and enabled better managements in such artificial lakes.

Primary production dynamics on the Agulhas Bank in autumn

The Agulhas Bank is a productive shelf sea, supporting important fish stocks, nursery grounds, and spawning sites. Few studies have examined the dynamics of primary production and the physio-chemical conditions that support this productivity during autumn. We report from a 14-day, 51-station survey of the central and eastern (21-27°E) Agulhas Bank in March 2019, during which we examined water-column structure, macronutrients, chlorophyll-a (total and size-fractionated), diatom cell counts and Net Primary Production (NPP). East to west trends were observed, with surface mixed layers (SML) and stratification increasing to the west. Euphotic zones were deeper than the SML, with SML irradiance conditions indicative of favorable light conditions for NPP. On average, surface waters contained ∼1.2 μmol N L−1 of nitrate (nitrate + nitrite; NO3) and ∼3 μmol Si L−1 of silicic acid, which contrasts with nutrient deficient subtropical source waters. Surface chlorophyll-a ranged from 0.3 to 5.1 mg m−3, with high values inshore and near the shelf break. Nanoplankton (2–20 μm) dominated size-fractionated chlorophyll-a, with microplankton (>20 μm) contributions increasing to the west. Measurements of NPP were collected at seven stations, ranging from 0.3 to 1.1 g C m−2 d−1, with a statistically significant relationship between integrated NPP and surface chlorophyll-a allowing further estimates of NPP (0.1–1.1 g C m−2 d−1). We estimated nitrogen-demand to support NPP, with a comparison to surface NO3 indicating ample nutrients to support daily NPP. Around half of the stations possessed a Subsurface Chlorophyll Maximum (SCM), with chlorophyll-a ranging from 1.7 to 10.3 mg m−3. Characteristics of the SCM (depth, light level, chlorophyll-to-carbon ratios) showed east to west variability, implying that the mechanisms of SCM formation ranged from in-situ growth (east) to photo-acclimation (west).

An 8-year record of phytoplankton productivity and nutrient distributions from surface waters of Saanich Inlet

Phytoplankton are the base of nearly all marine food webs and mediate the interactions of biotic and abiotic components in marine systems. Understanding the spatial and temporal changes in phytoplankton growth requires comprehensive biological, physical, and chemical information. Long-term datasets are an invaluable tool to study these changes, but they are rare and often include only a small set of measurements. Here, we present biological, physical and chemical oceanographic data measured periodically between March 2010 and November 2017 from the euphotic zone of Saanich Inlet, a temperate fjord on the west coast of British Columbia, Canada. The dataset includes measurements of dissolved macronutrients, total and size-fractionated chlorophyll-a, particulate carbon, nitrogen and biogenic silica, and carbon and nitrate uptake rates. This collection describes phytoplankton dynamics and the distribution of biologically-available macronutrients over time in the upper water column of Saanich Inlet. We establish a baseline for future investigations in Saanich Inlet and provide a data collection protocol that can be applied to similar productive coastal regions.

Factors Regulating the Relationship Between Total and Size-Fractionated Chlorophyll-a in Coastal Waters of the Red Sea

Phytoplankton biomass and size structure are recognized as key ecological indicators. With the aim to quantify the relationship between these two ecological indicators in tropical waters and understand controlling factors, we analyzed the total chlorophyll-a concentration, a measure of phytoplankton biomass, and its partitioning into three size classes of phytoplankton, using a series of observations collected at coastal sites in the central Red Sea. Over a period of 4 years, measurements of flow cytometry, size-fractionated chlorophyll-a concentration, and physical-chemical variables were collected near Thuwal in Saudi Arabia. We fitted a three-component model to the size-fractionated chlorophyll-a data to quantify the relationship between total chlorophyll and that in three size classes of phytoplankton [pico- (<2 μm), nano- (2–20 μm) and micro-phytoplankton (>20 μm)]. The model has an advantage over other more empirical methods in that its parameters are interpretable, expressed as the maximum chlorophyll-a concentration of small phytoplankton (pico- and combined pico-nanophytoplankton, and , respectively) and the fractional contribution of these two size classes to total chlorophyll-a as it tends to zero (Dp and Dp,n). Residuals between the model and the data (model minus data) were compared with a range of other environmental variables available in the dataset. Residuals in pico- and combined pico-nanophytoplankton fractions of total chlorophyll-a were significantly correlated with water temperature (positively) and picoeukaryote cell number (negatively). We conducted a running fit of the model with increasing temperature and found a negative relationship between temperature and parameters and and a positive relationship between temperature and parameters Dp and Dp,n. By harnessing the relative red fluorescence of the flow cytometric data, we show that picoeukaryotes, which are higher in cell number in winter (cold) than summer (warm), contain higher chlorophyll per cell than other picophytoplankton and are slightly larger in size, possibly explaining the temperature shift in model parameters, though further evidence is needed to substantiate this finding. Our results emphasize the importance of knowing the water temperature and taxonomic composition of phytoplankton within each size class when understanding their relative contribution to total chlorophyll. Furthermore, our results have implications for the development of algorithms for inferring size-fractionated chlorophyll from satellite data, and for how the partitioning of total chlorophyll into the three size classes may change in a future ocean.

Comparison of Two Methods to Assess the Size Structure of Phytoplankton Community Assemblages, in Liaodong Bay, China

Liaodong Bay is one of the largest semi-enclosed bays located in Bohai Sea, in northeast China. It is an important feeding and spawning place, even wintering and breeding migration base for many marine organisms. The size structure of phytoplankton communities is a key feature of marine ecosystems and can be used as an indicator for algae disaster, but the methods to evalute is still challenging. Here, we compared the size-fractionated chlorophyll-a method with a high-throughout sequencing method in an evaluation of the size structure of phytoplankton assemblages in Liaodong Bay, China, in 2014. The proportion of picophytoplankton was estimated at just 19% according to the results based on size-fractionated chlorophyll-a, which likely represents a severe underestimation. The sequencing method confirmed that many kinds of phytoplankton species appeared in more than one size-fractionated filter membrane, with some even dominating where the filter membrane pore size was greater than the individual phytoplankton size. According to the sequencing method, the phytoplankton assemblages in Liaodong Bay were dominated by picophytoplankton (average proportion = 44%), followed by the nanophytoplankton (average 32%), a composition that is more consistent with other studies. The sequencing method provides a more robust way to assess the size structure and taxonomic diversity of marine phytoplankton communities.

Three-dimensional coupling between size-fractionated chlorophyll-a, POC and physical processes in the Taiwan Strait in summer

Coastal water masses are inherently diverse, often with multiple water sources of different origins. Knowledge of characteristic salinity and temperature fingerprints can be very useful to differentiate between component water masses. For example, summertime river plumes carry relatively warm and less saline water into coastal waters, along with nutrients, clastic and organic sediments, and terrestrial-sourced biogenic particles. In an upwelling regime, relatively colder water carries marine-sourced nutrients and particles in water masses from deeper depths offshore. These two regimes are subject to biological-physical coupling that affects their size-differentiated particulate chlorophyll-a and organic carbon contents, and we show here how both regimes can be identified in these terms in the Taiwan Strait in summer. A close examination shows that microphytoplankton is the major contributor to chlorophyll-a concentrations in both regimes at all depths, and picophytoplankton is a minor contributor in the subsurface cold-water regime. The contribution from nanophytoplankton is relatively insignificant. Shipboard hydrographic profiling and water sampling was conducted over 28 h at a fixed location off the mouth of the Minjiang River in the northern Taiwan Strait, where the river plume and upwelling regimes overlap. Water samples were filtered onboard using a nested filtration system to separate suspended particles into >153, 63–153, 10–63, and 0.7–10 μm size classes. Our findings show that in the surface nepheloid layer, the river plume accounted for the physical process affecting large (>10 μm) terrestrial-sourced non-phytoplankton POC (organic debris), with an average C/N ratio of 92.2. The Upwelling regime accounted for the physical process affecting marine-sourced POC <10 μm containing fluorescence. In the intermediate nepheloid layer, which coincided with the subsurface chlorophyll maximum layer, most large (>10 μm) POC that contained fluorescence were either upwelling- or offshore-sourced, with an average C/N ratio of 8.5. In the benthic nepheloid layer, terrestrial- and marine-sourced POC containing fluorescence of all sizes were associated with river plume and upwelling regimes. Large (>63 μm) non-phytoplankton organic particles were also present, with an average C/N ratio of 10.2. Both spatial and temporal datasets point to strong three-dimensional coupling between biogenic particles and physical processes. The river plume and upwelling regimes further differentiate size-fractionated chlorophyll-a and POC.

Spatial variability of phytoplankton in the Pacific western boundary currents during summer 2014

The spatial distribution of phytoplankton was investigated during the summer of 2014 in two different regions of the Pacific western boundary current, namely the Warm Pool near the equator and the subtropical Kuroshio south area. Traditional approaches (size-fractionated chlorophyll-a (Chl-a) and microscopic analyses) combined with single-cell analysis (using a flow cytometer) were used to analyse the whole range of phytoplankton. Flow cytometry analysis resolved five clusters, two belonging to the pico-size fraction and three belonging to the nano-size fraction. Microscopy analysis revealed that the genera Coscinodiscus, Rhizosolenia, Chaetoceros and Ceratium were numerically dominant in the region studied. The lowest values of Chl-a, phytoplankton abundance and carbon biomass were found in the Kuroshio south area. Both Chl-a concentration data and flow cytometry analysis revealed that picophytoplankton were the predominant contributors to phytoplankton in the Pacific western boundary currents. Along the three transects during the summer cruise, Synechococcus and nanocyanobacteria-like organisms numerically dominated in surface waters with higher temperature. In contrast, eukaryotes were primarily distributed in subsurface waters with higher nutrients, especially in the eddy upwelling region mainly associated with the North Equatorial Counter Current. The vertical distribution of phytoplankton over the epipelagic layer reinforces the important role of currents in the north-western Pacific during summer.

Remote sensing of size structure of phytoplankton communities using optical properties of the Chukchi and Bering Sea shelf region

Abstract. Recent ocean warming and subsequent sea ice decline resulting from climate change could affect the northward shift of the ecosystem structure in the Chukchi Sea and Bering Sea shelf region (Grebmeier et al., 2006b). The size structure of phytoplankton communities provides an index of trophic levels that is crucial to understanding the mechanisms underlying such ecosystem changes and their implications for the future. This study proposes a new ocean color algorithm for deriving this characteristic by using the region's optical properties. The size derivation model (SDM) estimates the phytoplankton size index FL on the basis of size-fractionated chlorophyll-a (chl-a) using the light absorption coefficient of phytoplankton, aph(λ), and the backscattering coefficient of suspended particles including algae, bbp(λ). FL was defined as the ratio of algal biomass attributed to cells larger than 5 μm to the total. It was expressed by a multiple regression model using the aph(λ) ratio, aph(488)/aph(555), which varies with phytoplankton pigment composition, and the spectral slope of bbp(λ), γ, which is an index of the mean suspended particle size. A validation study demonstrated that 69% of unknown data are correctly derived within FL range of ±20%. The spatial distributions of FL for the cold August of 2006 and the warm August of 2007 were compared to examine application of the SDM to satellite remote sensing. The results suggested that phytoplankton size was responsive to changes in sea surface temperature. Further analysis of satellite-derived FL values and other environmental factors can advance our understanding of ecosystem structure changes in the shelf region of the Chukchi and Bering Seas.

Phytoplankton productivity and community structure in the vicinity of the Crozet Plateau during austral summer 2004/2005

The CROZet natural iron bloom and EXport experiment (CROZEX) was carried out around the Crozet Plateau (Southern Ocean) in late austral spring-early summer 2004/2005. The primary aim of this study was to test the hypothesis that natural Fe fertilisation from the islands enhanced primary production to the north of the plateau, leading to elevated carbon export. During the 12-week shipboard study (November–January), size-fractionated chlorophyll-a (chl-a), primary productivity and phytoplankton pigments were determined at 20 stations around the Crozet Plateau, both inside and outside the expected bloom region. Satellite-derived chl-a data from August 2004 to April 2005 revealed that the cruise period covered the declining stage of a large phytoplankton bloom north of the Crozet Plateau. Productivity derived from satellite chl-a data showed a strong north–south gradient, with productivity reaching a maximum of 1.9 g C m?2 d?1 in the north (late October), 0.6 over the plateau (late March) and 0.4 in the south (early December). From November to early December, measured chl-a and productivity were still relatively high northwest of the plateau (54–74 mg chl-a m?2 and 0.5–1.1 g C m?2 d?1), the assemblage being dominated by Phaeocystis sp. and microplankton representing 50% of total chl-a. Declining biomass and productivity to the northwest in mid-December was associated with a picoplankton (52%) dominated community (19–38 mg chl-a m?2 and 0.1–0.4 g C m?2 d?1). In contrast, at the north eastern edge of the plateau, a mixed diatom/Phaeocystis dominated bloom (229 mg chl-a m?2 and 3 g C m?2 d?1) developed in mid-January. South of the plateau in the control high-nutrient low-chlorophyll (HNLC) region, biomass and productivity were always significantly lower, with only a small chl-a peak in December, and a community composed of microflagellates, cyanobacteria and some giant diatoms. Particulate organic carbon (POC):chl-a ratios provided physiological evidence for the impact of Fe fertilisation north of the plateau. The assimilation rate (i.e. integrated productivity normalised to integrated chl-a, ?P/?chl-a) did not show significant differences between the north and south, probably due to the late sampling period relative to the bloom peak. Overall, measurements of primary productivity during CROZEX supported the hypothesis of enhanced biomass and production due to natural iron enrichment of the region north of the Crozet Plateau, while south of the plateau, low biomass and productivity resulted primarily from Fe limitation.

Comparison of environmental conditions in two representative oyster farming areas: Hiroshima Bay, western Japan and Oginohama Bay (a branch of Ishinomaki Bay), northern Japan

Sea water environmental conditions over annual cycles were investigated and compared between two oyster farming areas, western Hiroshima Bay and Oginohama Bay (a branch of Ishinomaki Bay) in Miyagi Prefecture, to appropriately manage oyster culture or more efficiently utilize farming areas. The environmental parameters of temperature, salinity, nutrient concentrations (NO2−N, NO3−N, NH4−N, PO4−P, and SiO2−Si) and size-fractionated chlorophyll-a ( 20μm), and abundances of microzooplankton were measured in each bay at the surface, and 2 and 5 m depth layers. Differences in the annual mean values and results with monthly paired Student’s t-tests showed that salinity was lower, and temperature, nutrient (especially PO4−P) and chlorophyll-a concentrations, and abundance of microzooplankton, were higher in Hiroshima Bay than in Oginohama Bay. Differences in environmental conditions between inshore and offshore areas of each bay suggest that inflows of river water in western Hiroshima Bay and sea water from offshore had the most significant effects on the environmental conditions. It is concluded that such oceanographic and biological differences strongly affect the oyster farming system, especially regarding the optimum usage of offshore areas in Summer under clean, cold and stable seawater conditions, rather than food quantity in Hiroshima Bay, and under more abundant food conditions in Oginohama Bay.

Seasonal Changes in Zooplankton Biomass and Grazing in a Temperate Estuary, South Africa

Seasonal changes in size-fractionated chlorophylla (chla) and primary production, zooplankton biomass and zooplankton community grazing were investigated at six stations in the temperate Kariega Estuary, South Africa. Variations in chla, primary production and zooplankton biomass during the study demonstrated a seasonal pattern with minimum values recorded in winter and maximum values in summer. Total chla and primary production during the study ranged from 0·11 to 2·01μg l−1and between 13·9 and 37·7mg Cm−3d−1, respectively. Throughout the study, picophytoplankton (<2μm) dominated total chla and primary production. Zooplankton biomass during the study ranged from 7·98 to 59·31mg Dwt m−3. Pearson correlation analysis indicated that total zooplankton biomass was significantly correlated to total chla concentration (P<0·05). Community grazing rates of the zooplankton also demonstrated a seasonal pattern and ranged from 0·91 to 6·93ml l−1h−1. The elevated grazing rates recorded during summer could be related to higher seawater temperatures and zooplankton biomass. Grazing impact of zooplankton community corresponded to a loss of between 1·4 and 57·8% (mean=9·8%) of the total daily phytoplankton production. Specific ingestion rates of the two dominant copepod species, Acartia longipatella and Pseudodiaptomus hessei, indicated that carbon derived from the consumption of phytoplankton alone was sufficient to meet the basic metabolic requirements of the two copepods during summer and during spring for A. longipatella. The low contribution of phytoplankton to the total daily ration of the two copepods appears to be related to the size structure of the local phytoplankton which is too small to be grazed efficiently by the two copepod species.

Green krill, the indicator of micro- and nano-size phytoplankton availability to krill

Size-fractionated chlorophyll-a concentrations of surface seawater were measured for pico-, nano-, and micro-size fractions ( 10 μm respectively) during commercial krill fishery operations in the waters north of the South Shetland Islands. The proportion of green krill (individuals discoloured due to active feeding on phytoplankton) had significant regressions with chlorophyll-a concentrations in micro- and nano-size fractions. Between these two fractions, chlorophyll-a concentration in the micro-size fraction showed the higher partial regression coefficient. This result shows the importance of phytoplankton larger than nano-phytoplankton, especially micro-phytoplankton, in terms of a phytoplanktonic food source for Antarctic krill in the natural environment.