Is it possible to derive accurately Total Suspended Matter concentration or its proxy, turbidity, from remote sensing data in tropical coastal lagoon waters? To investigate this question, ...hyperspectral remote sensing reflectance, turbidity and chlorophyll pigment concentration were measured in three coral reef lagoons. The three sites enabled us to get data over very diverse environments: oligotrophic and sediment-poor waters in the southwest lagoon of New Caledonia, eutrophic waters in the Cienfuegos Bay (Cuba), and sediment-rich waters in the Laucala Bay (Fiji). In this paper, optical algorithms for turbidity are presented per site based on 113 stations in New Caledonia, 24 stations in Cuba and 56 stations in Fiji. Empirical algorithms are tested at satellite wavebands useful to coastal applications. Global algorithms are also derived for the merged data set (193 stations). The performances of global and local regression algorithms are compared. The best one-band algorithms on all the measurements are obtained at 681 nm using either a polynomial or a power model. The best two-band algorithms are obtained with R412/R620, R443/R670 and R510/R681. Two three-band algorithms based on Rrs620.Rrs681/Rrs412 and Rrs620.Rrs681/Rrs510 also give fair regression statistics. Finally, we propose a global algorithm based on one or three bands: turbidity is first calculated from Rrs681 and then, if < 1 FTU, it is recalculated using an algorithm based on Rrs620.Rrs681/Rrs412. On our data set, this algorithm is suitable for the 0.2-25 FTU turbidity range and for the three sites sampled (mean bias: 3.6 %, rms: 35%, mean quadratic error: 1.4 FTU). This shows that defining global empirical turbidity algorithms in tropical coastal waters is at reach.
Data pertaining to environmental conditions, sympagic (sea ice) microalgal dynamics and particle flux were collected before the spring ice break-up 2001 in Pierre Lejay Bay, adjacent to the Dumont ...d'Urville Station, Petrel Island, East Antarctica. An array of two multiple sediment traps and a current meter was deployed for five weeks, from 8 November to 6 December 2001. The sea-ice chlorophyll a and particulate organic carbon (POC) averaged 0.6 mg l
−1
(30 mg m
−2
) and 20 mg l
−1
(1 g m
−2
) near the coast. The POC export flux that reached a maximum of 79 mg m
−2
d
−1
during the study period was high compared to the one for the Weddell Sea. The flux was homogeneous from the surface to 47 m depth and increased sharply 33 days before the effective ice break-up. A north-western progressive vector of currents (i.e., Lagrangian drift) in the sub-ice surface waters was demonstrated. Bottom ice, platelet ice and under-ice water at 5 m were characterized by differences in colonization and short-term succession of microalgae.
Phytoplankton growth and microzooplankton grazing rates were investigated using the seawater dilution technique during a French Joint Global Ocean Flux Study cruise focusing on grazing processes in ...the high‐nutrient, low‐chlorophyll equatorial Pacific at 180° (Etude du Broutage en Zone Equatoriale, October–November, 1996). Raw rate estimates based on spectrofluorometric and high‐performance liquid chromatography pigment analyses were typically in close agreement, but most showed substantial imbalances in growth and grazing. Flow cytometric (FCM) analyses were used both as an alternate approach for distinguishing populations and as a means for adjusting pigment‐based growth estimates for changes in cellular chlorophyll content and biovolume. Total chlorophyll a (Tchl a) gave mean community growth rates of 0.76 d−1 at 30 m and 0.27 d−1 at 60 m. Grazing rates averaged 0.56 and 0.15 d−1 at the two depths, respectively, and 69% of phytoplankton growth overall. For the prokaryotic picophytoplankter, Prochlorococcus (PRO), rate estimates from dv‐chl a and FCM cell counts generally indicated balanced growth and grazing and therefore close grazing control by microzooplankton. At the equator, rate estimates from dv‐chl a averaged 0.6–0.7 d−1 at 30 m and 0.25–0.26 at 60 m and were consistent with inferences based on diel pigment variations in the 30–70 m depth range. Phytoplankton production estimates from experimentally determined rates and microscopical assessments of autotrophic carbon at 30 m (mean = 19 mg C m−3 d−1) agreed well with contemporaneous measurements by 14C uptake. Diatom growth rate estimates (1.0–1.6 d−1), constrained by contemporaneous measurements of silicate uptake, implied a relatively small biomass (10–45 nmol C L−1) with high rates of turnover and recycling.
The vertical structure and chlorophyllous pigments of picophytoplanktonic populations of the northeastern Atlantic Ocean were studied by flow cytometry and spectrofluorometry. Three sites (EU, MESO ...and OLIGO) on a hydrological gradient from near coastal, eutrophic waters to offshore, oligotrophic waters were occupied in October 1991 (except EU), June 1992 and December 1992. The population structure of the EU site (20°32′N 18°34′W) was greatly influenced by a permanent, wind-induced upwelling. The latter was well developed in June, and an important nutrient enrichment of surface waters ensued (> 10 mM NO
2+NO
3). The latter favored the blooming of diatoms, but picophytoplanktonic populations remained low. In December the upwelling was less developed, and there was a dramatic increase of the cell abundances of both prokaryotic (
Prochlorococcus and
Synechococcus) and picoeukaryotic populations at the EU site. Cells of all groups were concentrated and homogeneously distributed in the upper, 30–35 m thick, mixed layer. Similar population structures were observed in June and December at the MESO site (18°29′N 21°05′W). In these cases, both
Synechococcus cyanobacteria (with concentrations of up to 5 × 10
5 cells ml
−1 during winter) and picoeukaryotes (typically 1–2 × 10
4 cells ml
−1) made significant contributions to the integrated picophytoplanktonic biomass in terms of carbon (166–333 μg C cm
−2 and 92–155 μg C cm
−2, respectively).
Prochlorococcus made a smaller contribution (6–48 μg C cm
−2), as also indicated by a low ratio of divinyl-chlorophyll
a to total chlorophyll
a (≤22%). The population structure observed in October at the MESO site was much more variable, even at the time-scale of hours. At the OLIGO site (21°02′N 31°08′W), the relative contribution of
Prochlorococcus to picophytoplankton carbon and total chlorophyll standing stocks increased dramatically (> 50%), mainly as a result of a sharp decrease of both
Synechococcus and picoeukaryotes cell concentrations down to a few thousands cells per ml. There was little seasonal change in the vertical structure of any of the three populations at this site. From analyses of cell cycle distributions during a 31 h time-series, growth rates were estimated for
Prochlorococcus as 0.41 day
−1 at mid-depth (80 m) and 0.39 day
−1 in the deep chlorophyll maximum.
In situ diel variations of extracted chlorophyllous pigments, beam attenuation by particles (cp), and in vivo chlorophyll fluorescence (Fiv) were investigated during a 5‐day time series in ...high‐nutrient, low‐chlorophyll waters of the equatorial Pacific (date line = 180°). Samples were taken hourly at 10 depths in the upper 100 m during the first 48 hours, then sampling frequency decreased to 3 hours. In the 30–70 m layer the integrated chlorophyll concentrations, cp, and Fiv increased during the light period, but the minima and, especially, maxima were not fully synchronized. The lowest values of total chlorophyll a (Tchl a = chlorophyll a + divinyl‐chlorophyll a) occurred around 5–6 hours, slightly (0–2 hours) before that of cp and Fiv. Tchl a reached a maximum around 1500 hours ± 1 hour, clearly before cp (1700 hours) and Fiv (1900 hours). In the 0–30 m layer, diel variations of the integrated chlorophyll concentrations, cp, and Fiv were clearly out of phase. They showed a nocturnal increase in Tchl a, starting around midnight and peaking in early morning (0900 hours). In contrast, cp increased only during the light period in the upper 30 m, and variations of Fiv were largely opposite to those of extracted Tchl a. Specific phytoplankton growth (μ0) and grazing loss (g) rates were estimated from diel variations in the 30–70 m layer and compared to independent rate estimates from experimental incubations. These results are discussed in the context of physical processes and physiological responses of the cells to the daily photocycle.
Under an apparent monotony characterized by low phytoplankton biomass and production, the Pacific equatorial system may hide great latitudinal differences in plankton dynamics. On the basis of 13 ...experiments conducted along the 180° meridian (8°S–8°N) from upwelled to oligotrophic waters, primary production was strongly correlated to chlorophyll a (chl a), and the productivity index PI (chl a‐normalized production rate) varied independently of macronutrient concentrations. Rates of total (14C uptake) and new (15N‐NO3 uptake) primary production were measured in situ at 3°S in nutrient‐rich advected waters and at 0° where the upwelling velocity was expected to be maximal. Primary production was slightly higher at the equator, but productivity index profiles were identical. Despite similar NO3 concentrations, new production rates were 2.6 times higher at 0° than at 3°S, in agreement with much higher concentrations of biogenic particulate silica and silicic acid uptake rates (32Si method) at the equator. Furthermore, phytoplankton carbon concentrations from flow cytometric and microscopical analyses were used with pigment and production values to assess C:chl a ratios and instantaneous growth rates (μ). Growth rates in the water column were significantly higher, and C:chl a ratios lower at 0° than at 3°S, which is consistent with the more proximate position of the equatorial station to the source of new iron upwelling into the euphotic zone. For the transect as a whole, compensatory (inverse) changes of C:chl a and μ in response to varying growth conditions appear to maintain a high and relatively invariant PI throughout the equatorial region, from high‐nutrient to oligotrophic waters.
As part of the French Joint Global Ocean Flux Study Etude du Broutage en Zone Equatoriale program, we investigated the distributions of microorganisms (bacteria and protists <200 μm) in the upper 120 ...m of the equatorial Pacific from 8°S to 8°N, along 180°. Population distributions, determined by a combination of flow cytometry, microscopy and spectrofluorometry, were closely related to physical features across the study site. Phytoplankton biomass, ranging from 1.2 to 34.2 μg C L−1 and averaging 15.5 μg C L−1, was most enhanced in the divergence zone. Carbon to chlorophyll ratios were also enhanced in the divergence zone and showed distinct latitudinal variations. Heterotrophic biomass, excluding ciliates, was patchy across the area, ranging from 5 to 36 μg C L−1 and averaging 13 μg C L−1. Prokaryotic species (Prochlorococcus spp., Synechococcus spp., and heterotrophic bacteria) showed similar patterns of abundance, with the main feature being their distributional asymmetry to the south of the equator. Both autotrophic and heterotrophic biomass were enriched in the convergent zone at 4°–5°N between the South Equatorial Current and the North Equatorial Counter Current. Heterotrophic biomass exceeded phytoplankton biomass in the more nutrient‐impoverished waters to the north and in the branch of a tropical instability wave eddy. Microplankton represented only a small portion of the total autotrophic carbon and was comprised mostly of dinoflagellates. Large species dominated the relatively modest diatom biomass. Food web interactions and biogeochemical fluxes in the central equatorial Pacific may be significantly influenced by temporal and spatial variability of the microbial community associated with physical features of the region.
Light-dependent variation in pigment content was examined in 11 marine phytoplankton species representing 8 algal classes. Batch cultures of each species were acclimated to 7 irradiances between 10 ...and 700 μmol photons m–2s–1. High-performance liquid chromatography analysis on exponentially growing cultures revealed that pigment ratios normalised to chlorophylla(chla) generally fell within the ranges previously reported for species of the same taxa. Unambiguous light-harvesting pigments (e.g. chlorophylls, fucoxanthin) showed limited amplitude of variation between low and high light conditions, while those involved in photoprotection (e.g. zeaxanthin, lutein, alloxanthin, diatoxanthin) dramatically increased under high light. Most of the relationships between pigment:chlaratios and growth irradiance were quite well described using simple linear models. Among pigments shared by several species, chlbandc₃, fucoxanthin, zeaxanthin, lutein, diadinoxanthin and diatoxanthin exhibited similar trends regardless of the species studied, allowing general equations to be determined for each of these pigments. Determination of light:pigment relationships is a prerequisite for the development of more advanced modeling techniques such as artificial neural networks in chemotaxonomic studies.
Contributions of filamentous and picoplanktonic cyanobacteria to the phytoplankton community structure were examined in New Caledonian waters during the 2001-2003 El Niño period at 2 ocean stations ...(Loyalty Channel and Santal Bay) and 1 coral-reef lagoon station (Ouinne). Morphometric characteristics of diazotrophic filamentous cyanobacteria are given, as well as the seasonal and inter-annual variations of their surface areas and integrated abundances. Trichodesmium tenue and T. thiebautii were the dominant species followed by T. erythraeum, altogether accounting for more than 51-80% of the biomass of the free-living filamentous cyanobacteria. Katagnymene spp. accounted for a smaller percentage (<13.8% at ocean stations, <3.6% in the lagoon). Richelia intracellularis biomass was relatively small (<1% of total surface area and volume of Trichodesmium trichomes), with the highest concentration observed in summer (735 trichomes l-1). Colonies of unidentified cyanobacteria composed of spherical cells accounted on average for <1% of the Trichodesmium biomass, with maximum values exceeding 4000 cells l-1. Abundance of filamentous cyanobacteria varied according to environmental factors; summer 2001-2002 was characterized by low filamentous diazotroph abundance, and summer 2003, at the peak of the 2001-2003 El Niño, was particularly rich in filamentous cyanobacteria (with a maximum Trichodesmium spp. abundance of 4500 trichomes l-1 in the Loyalty Channel). A similar variability pattern was observed for large diatoms and dinoflagellates, and for all picoeukaryotic populations. Different biomass estimators are provided, including cell abundances, pigment concentrations including chlorophylls and phycoerythrin, and carbon content.
Diel variations of copepod biomass and feeding were studied at two time series stations in the high‐nutrient, low‐chlorophyll region of the central equatorial Pacific (0° and 3°S, 180°). During ...48‐hour studies at each station, samples were taken at 3‐hour frequency in the neuston layer (0–1 m) and over the 0–100 m depth range. Feeding rates were assessed through the spectrofluorometric analysis of gut pigment contents. During the cycles, significant variations of biomass and gut pigment contents were found in both the neuston layer and the 0–100 depth strata. However, the amplitude of day‐night variations of feeding was larger and more clearly related to the day‐night cycle in the neuston layer than in the water column. Maxima in biomass and gut pigment contents did not coincide in either the surface or the water column. The differences in the mean copepod feeding activities at the equator and at 3°S were related to the mean standing stocks of chl a and to the pattern of undegraded pigments, which were different at the surface and in the water column, especially at the equator. In the surface layer, gut pheopigment minima were observed during the day, and maxima at night. This pattern clearly followed the periodicity of diel vertical migration, with intense feeding observed after the main upward ascent around sunset. Conversely, the feeding pattern in the integrated water column was more related to food abundance with a higher feeding during the day and around sunset, especially at the equator. Gut pigment contents in the water column displayed late afternoon maxima and dawn minima, coinciding with the maxima and minima of depth‐integrated chlorophyll a (chl a). On the basis of standing stocks of chl a containing particles larger than 3 μm, mean grazing pressure (i.e., specific grazing rates multiplied by copepod dry weights and divided by in situ chl a) varied between 3.0 and 4.0% in the upper 100 m and between 3.5 and 3.8% in the neuston layer. Because of the cyclical nature of feeding activities, short‐term variability in grazing pressure estimates were substantial, with individual estimates ranging from a high of 17.5% d−1 to a low of 0.2% d−1 in the neuston layer and from 0.9 to 7.8% d−1 in the 0–100 m water column.
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