Search

Published more than 60 years ago in this Journal, the article in which Sverdrup proposed the concept of critical depth to explain the initiation of the spring bloom in the North Atlantic has accrued an exceptionally large number of citations and continues to be cited more than 50 times per year. The framework provided by Sverdrup has now been applied, adapted, and tested across many aquatic systems worldwide. Datasets have been collected; models have been built on the framework: these studies have generated new insights into phytoplankton dynamics and interesting debates on the relative importance of the various factors responsible for phytoplankton blooms. This article theme set presents some of the most recent efforts to discuss and test Sverdrup's critical depth hypothesis using a diverse set of approaches, ranging from controlled experiments to field observations as well as numerical and analytical models. The set of papers celebrates an elegant and powerful hypothesis that has had long-lasting influence. It is to be expected that it will also stimulate future research, adding even more to our understanding of one of the most fundamental processes in biological oceanography.

Racault, Marie-Fanny LP/0000-0002-7584-2515; Agirbas, Ertugrul/0000-0001-7987-9668; Martinez-Vicente, Victor/0000-0003-3492-583X; Brewin, Robert/0000-0001-5134-8291 WOS: 000336490000003 Ecological indicators are used extensively as tools to manage environmental resources. in the oceans, indicators of plankton can be measured using a variety of observing systems including: mooring stations, ships, autonomous floats and ocean colour remote sensing. Given the broad range of temporal and spatial sampling resolutions of these different observing systems, as well as discrepancies in measurements obtained from different sensors, the estimation and interpretation of plankton indicators can present significant challenges. To provide support to the assessment of the state of the marine ecosystem, we propose a suite of plankton indicators and subsequently classify them in an ecological framework that characterizes key attributes of the ecosystem. We present two case studies dealing with plankton indicators of biomass, size structure and phenology, estimated using the most spatially extensive and longest in situ and remote-sensing observations. Discussion of these studies illustrates how some of the challenges in estimating and interpreting plankton indicators may be addressed by using for example relative measurement thresholds, interpolation procedures and delineation of biogeochemical provinces. We demonstrate that one of the benefits attained, when analyzing a suite of plankton indicators classified in an ecological framework, is the elucidation of non-trivial changes in composition, structure and functioning of the marine ecosystem. EC FP7 GreenSeas project [265294 FP7-ENV-2010]; Natural Environment Research CouncilNERC Natural Environment Research Council [pml010008] Funding Source: researchfish; Directorate For GeosciencesNational Science Foundation (NSF)NSF - Directorate for Geosciences (GEO) [1154661] Funding Source: National Science Foundation; Division of Ocean SciencesNational Science Foundation (NSF)NSF - Directorate for Geosciences (GEO) [1154661] Funding Source: National Science Foundation The work was funded by the EC FP7 GreenSeas project (265294 FP7-ENV-2010).

Stuart, V., Platt, T., and Sathyendranath, S. 2011. The future of fisheries science in management: a remote-sensing perspective. – ICES Journal of Marine Science, 68: 644–650. Earth observation from satellites offers vast potential for fisheries applications, including management of marine resources, stock assessment, marine aquaculture, and fish harvesting. One of the most promising avenues for the use of satellite data for fisheries science in management lies in quantifying objectively the variables that result in large and small year classes of exploited stocks. The influence of fluctuations in the availability of food in the critical period of larval stages can be investigated through the application of ecological indicators describing the variability of the pelagic ecosystem at a given time and place. These indices can increase our understanding of the relationship between ecosystem factors and the recruitment of key species. Despite the many demonstration applications published to date, little use is being made of satellite data to support fisheries science in management. We discuss some of the obstacles that lie in the way of the operational use of satellite data and suggest actions that could facilitate its broader application.

Zhai, L., Platt, T., Tang, C., Sathyendranath, S., and Hernández Walls, R. 2011. Phytoplankton phenology on the Scotian Shelf. – ICES Journal of Marine Science, 68: . The impact of physical forcing on seasonal and interannual phytoplankton dynamics was examined using SeaWiFS chlorophyll, AVHRR sea surface temperature (SST), nitrate, and other hydrographic measurements for the Scotian Shelf and Scotian Slope. The spring bloom was characterized by a shifted Gaussian function fitted to seasonal chlorophyll time-series. The background chlorophyll (a constant term in the Gaussian function) is a joint property of the stratification and bio-optics of the mixed layer. Rapid shoaling of the mixed-layer depth in spring promoted the early spring bloom on the middle Scotian Shelf and Slope, triggered when averaged light in the mixed layer reached 15 W m−2. The duration of the spring bloom was prolonged in slope water, resulting in a discontinuity in duration between the shelf and slope water masses. The position of the latitudinal discontinuity in duration was correlated with that of the shelf–slope front in SST. The amplitude of the spring bloom was correlated with the nitrate inventory in the surface layer at the end of winter. The rate of decrease (increase) in chlorophyll after the spring bloom was related to the depletion (resupply) of nitrate in summer. The position of the shelf–slope front influenced the interannual variability of bloom characteristics.

Forget, M-H., Platt, T., Sathyendranath, S., and Fanning, P. 2011. Phytoplankton size structure, distribution, and primary production as the basis for trophic analysis of Caribbean ecosystems. – ICES Journal of Marine Science, 68: 751–765. An oceanographic survey was conducted in Caribbean waters in April and May 2006 to assess the marine ecosystem in the context of a trophic analysis of the foodweb. Analyses of pigment and absorption data revealed that the phytoplankton community was dominated by pico- and nanophytoplankton, particularly at the deep chlorophyll maximum. Based on cluster analysis of remotely sensed data, three dynamic provinces were defined for the region. A 5-year time-series of sea surface temperature and chlorophyll concentration provided information on the annual cycle of these properties. To implement the computation of primary production on a synoptic scale, parameters characterizing the biomass profiles and photosynthesis–irradiance relationships were assigned using four protocols: two regional approaches, a regression with surface chlorophyll, and the nearest-neighbour method (NNM), which has the advantage of assigning parameters on a pixel-by-pixel basis. Monthly images of primary production were computed over an annual cycle using MODIS chlorophyll a concentration. The NNM and the use of dynamic provinces emerged as the methods of choice for parameter assignment. Finally, a new approach from remotely sensed data was developed to estimate production-to-carbon ratios, a key input to Ecopath models. The results contribute to ecotrophic analysis of the Lesser Antilles Pelagic Ecosystem project.

Ouellet, P., Fuentes-Yaco, C., Savard, L., Platt, T., Sathyendranath, S., Koeller, P., Orr, D., and Siegstad, H. 2011. Ocean surface characteristics influence recruitment variability of populations of northern shrimp (Pandalus borealis) in the Northwest Atlantic. – ICES Journal of Marine Science, 68: 737–744. Remotely sensed data were used to derive simple ecosystem indicators for four regions of the Northwest Atlantic to test the hypothesis that sea surface temperatures (SSTs) and spring phytoplankton bloom characteristics (initiation, timing, intensity, and duration) have a significant influence on larval survival and recruitment of northern shrimp (Pandalus borealis). For all years (1998–2007) and regions, hatching was after the initiation of the bloom and before or after the bloom reached its maximum intensity. The results suggest that the best survival of larvae is associated with high warming rates of SST following hatching, but in very cold environments, warm temperatures at hatching seem to be important for larval survival. The analyses also indicate that larval survival is supported by an early, long phytoplankton bloom which attains high concentrations of chlorophyll a. The results demonstrate the potential of remotely sensed data for deriving simple population-specific ecosystem indicators for potential use in building operational recruitment models for predicting changes in northern shrimp abundance.

Roy, S., Platt, T., and Sathyendranath, S. 2011. Modelling the time-evolution of phytoplankton size spectra from satellite remote sensing. – ICES Journal of Marine Science, 68: 719–728. A dynamic size-structured model is developed for phytoplankton and nutrients in the oceanic mixed layer and applied to extract phytoplankton biomass at discrete size fractions from remotely sensed, ocean-colour data. General relationships between cell size and biophysical processes (such as sinking, grazing, and primary production) of phytoplankton were included in the model through a bottom–up approach. Time-dependent, mixed-layer depth was used as a forcing variable, and a sequential data-assimilation scheme was implemented to derive model trajectories. From a given time-series, the method produces estimates of size-structured biomass at every observation, so estimates seasonal succession of individual phytoplankton size, derived here from remote sensing for the first time. From these estimates, normalized phytoplankton biomass size spectra over a period of 9 years were calculated for one location in the North Atlantic. Further analysis demonstrated that strong relationships exist between the seasonal trends of the estimated size spectra and the mixed-layer depth, nutrient biomass, and total chlorophyll. The results contain useful information on the time-dependent biomass flux in the pelagic ecosystem.

Platt, T., Sathyendranath, S., and Fuentes-Yaco, C., 2007. Biological oceanography and fisheries management: perspective after 10 years. – ICES Journal of Marine Science, 64: 863–869. Despite 100 years of research into the relationship between oceanographic factors and fish recruitment, it has proved very difficult to demonstrate causal connections between properties of the marine ecosystem and the success of fisheries. Some authors have been led to conclude that such causal connections, therefore, do not exist: a corollary would be that biological oceanography is of limited relevance to fisheries issues. However, it would be premature to dismiss biological oceanography as a tool in fisheries management. If we have not been able to implicate ecosystem factors as a significant source of variance in fish recruitment, it may be because the search has been conducted at an inappropriate scale, a consequence of the limitations of ships as oceanographic platforms. The advent of remotely sensed data from satellites greatly extends the scales of time and space at which synoptic oceanography can be carried out. Access to such data allows a wider range of hypotheses to be tested, than is possible with ships alone, on the relationship between ecosystem factors and recruitment. Applications in both the Atlantic and Pacific have demonstrated strong fluctuations between years in the timing and the intensity of phytoplankton dynamics, with implications for recruitment and growth of exploited populations of fish and invertebrates. The results provide essential intelligence for those charged with stewardship of the marine environment.

The relationship between sea ice and the phytoplankton spring bloom over the Newfoundland and Labrador shelves is examined using remotely-sensed chlorophyll data and sea-ice data for the period 1998–2004. A regression analysis between the two data sets shows that the retreat of sea ice precedes the spring bloom, and the inter-annual variation of the spring bloom is closely correlated with the start time of ice retreat. The spring bloom off Canada’s east coast usually starts on the eastern Grand Banks. Here, the water properties are strongly influenced by sea ice on the Newfoundland shelves in early spring when accelerated ice melting causes the ice edge to retreat north and the melt water is advected south by the Labrador Current. After the ice retreat, the water on the eastern Grand Banks is rapidly stratified and the mixed layer shallows as a result of surface freshening. The shallow mixed layer promotes phytoplankton growth. The regression analysis also reveals that an early spring bloom or ice retreat tends to prolong the duration of the spring bloom.

A method is presented for extraction of the photosynthesis-response parameters from profiles of phytoplankton production. The procedure, previously proposed but not tested, is implemented here in various types of aquatic system and a protocol is established for its use. Values of daily primary production integrated over the photic zone were estimated from in situ or simulated in situ incubations in four coastal and open-ocean marine systems, and from photosynthesis-irradiance (P‐E) curves in the afore-mentioned marine systems, as well as in two freshwater systems. The slope of the measured daily water-column production (normalised to water-column chlorophyll a biomass) plotted against the daily incident irradiance was variable from system to system (0.09 to 0.60), showing a broader range than previously reported values. Using an iterative procedure, we estimated the photosynthetic parameters from this linear relationship. Generally, estimated values lie within the 95% confidence interval of the photosynthetic parameters obtained from the P‐E curves, showing that the estimates agree well with measurements. The new method, based on the photophysiological response of the phytoplankton community, provides a way to enhance our ability to compute primary production from remote sensing of ocean colour.

Stuart, V., Platt, T., Sathyendranath, S., and Pravin, P. 2011. Remote sensing and fisheries: an introduction. – ICES Journal of Marine Science, 68: 639–641. The international coordination project SAFARI (Societal Applications in Fisheries and Aquaculture using Remotely-sensed Imagery) organized a symposium on Remote Sensing and Fisheries in Kochi, India, 11–17 February 2010. The well-attended symposium highlighted various applications of remote sensing to fisheries and aquaculture and identified various steps that would further enhance the use of remote sensing for sustainable management of marine resources and stewardship of the oceans.

Data from three cruises (Arabesque 1 and 2 cruises in the Arabian Sea and the Vancouver Island cruise) were examined to assess the importance of species composition and accessory pigments in modifying specific absorption coefficients. The three cruises differed widely in their phytoplankton assemblages with small cells dominating the Arabesque 2 cruise and large diatoms the Vancouver Island cruise. Absorption spectra from each cruise were decomposed into 13 Gaussian bands repre- senting absorption by the major chlorophylls and accessory pigments. The maximum specific peak height (p£) for each Gaussian band was obtained by regressing Gaussian peak heights against the concentration of the pigment responsible for the absorption band. This relationship was generally non-linear and was fitted with a rectangular hyperbolic function. Changes in the maximum specific peak heights between cruises reflect changes in the packaging effect, which were most apparent at Gaussian bands of high absorption (400-490 nm), but were close to zero at the Gaussian band centered around 623 nm, associated with chlorophyll a (Chi a). The maximum specific peak height of this Gaussian band, p£(623), may be used to obtain reliable estimates of Chi a for any phytoplankton assemblage, unaffected by variations caused by the package effect. Comparing the Arabesque 2 peak heights with the Vancouver data, an apparent flattening effect of 0.42 at 440 nm and 0.62 at 676 nm was found for the Vancouver data relative to the Arabesque 2 data (assuming a zero flattening effect for the latter). Multiple linear regression analysis suggested that 29-42% of the variability in the specific absorption coefficient of phytoplankton at 440 nm was due to changes in pigment composi- tion, while the remaining 58-71% was due to changes in the package effect. An inverse relationship was found between the proportion of non-photosynthetic pigments (NPC) and ambient Chi a concen- tration, suggesting that small cells (generally found in oligotrophic waters) had higher proportions of photoprotective pigments.

Variations in the shapes of the action and absorption spectra, and the physiological parameters of phytoplankton (a", the biomass (B)-specific initial slope of the photosynthesis-irradi- ance curve; PS,, the plateau of the curve; m, the maximum quantum yield of photosynthesis), were studied. Data were collected in the North Atlantic in five biogeochemical provinces for two seasons: in fall of 1992 and spring of 1993. Further, some nine independent variables were tested for their ability to predict the physiological parameters, using linear regression analyses. It was found that the parameters were more variable between seasons than between provinces, although the differences in the spectral shapes between provinces were significant. It was also shown that several independent variables (alone or in combination) were able to predict a significant fraction of the total variance in the parameters. However, the correlation between variables and parameters differed unsystemati- cally both between provinces and seasons. The results suggest that predicting a parameter from an independent variable would have to be carried out at a province level: the relationship changed or disappeared when data from two or more provinces were pooled.

An estimate of global net primary production in the ocean has been computed from the monthly mean near-surface chlorophyll fields for 1979-1986 obtained by the Nimbus 7 CZCS radiometer. Our model required information about the subsurface distribution of chlorophyll, the parameters of the photosynthesis-light relationship, the sun angle and cloudiness. The computations were partitioned among 57 biogeochemical provinces that were specified from regional oceanography and by examination of the chlorophyll fields. Making different assumptions about the overestimation of chlorophyll by the CZCS in turbid coastal areas, the global net primary production from phytoplankton is given as 45-50 Gt C year"1. This may be compared with current published estimates for land plants of 45-68 Gt C year"' and for coastal vegetation of 1.9 Gt C year"1.