Graham Savidge

The Portugal coastal counter current off NW Spain: new insights on its biogeochemical variability

Abstract Time series of wind-stress data, AVHRR and SeaWiFS satellite images, and in situ data from seven cruises are used to assemble a coherent picture of the hydrographic variability of the seas off the Northwest Iberian Peninsula from the onset (September–October) to the cessation (February–May) of the Portugal coastal counter current (PCCC). During this period the chemistry and the biology of the shelf, slope and ocean waters between 40° and 43°N have previously been undersampled. Novel information extracted from these observations relate to: 1. The most frequent modes of variability of the alongshore coastal winds, covering event, seasonal and long-term scales; 2. The conspicuous cycling between stratification and homogenisation observed in PCCC waters, which has key implications for the chemistry and biology of these waters; 3. The seasonal evolution of nitrite profiles in PCCC waters in relation to the stratification cycle; 4. The Redfield stoichiometry of the remineralisation of organic matter in Eastern North Atlantic Central Water (ENACW)—the water mass being transported by the PCCC; 5. The separation of coastal (mesotrophic) from PCCC (oligotrophic) planktonic populations by a downwelling front along the shelf, which oscillates to and fro across the shelf as a function of coastal wind intensity and continental runoff; and 6. The photosynthetic responses of the PCCC and coastal plankton populations to the changing stratification and light conditions from the onset to the cessation of the PCCC.

Diatom carbon export enhanced by silicate upwelling in the northeast Atlantic

Diatoms are unicellular or chain-forming phytoplankton that use silicon (Si) in cell wall construction. Their survival during periods of apparent nutrient exhaustion enhances carbon sequestration in frontal regions of the northern North Atlantic. These regions may therefore have a more important role in the ‘biological pump’ than they have previously been attributed, but how this is achieved is unknown. Diatom growth depends on silicate availability, in addition to nitrate and phosphate, but northern Atlantic waters are richer in nitrate than silicate. Following the spring stratification, diatoms are the first phytoplankton to bloom. Once silicate is exhausted, diatom blooms subside in a major export event. Here we show that, with nitrate still available for new production, the diatom bloom is prolonged where there is a periodic supply of new silicate: specifically, diatoms thrive by ‘mining’ deep-water silicate brought to the surface by an unstable ocean front. The mechanism we present here is not limited to silicate fertilization; similar mechanisms could support nitrate-, phosphate- or iron-limited frontal regions in oceans elsewhere.

Mixing and phytoplankton growth around an island in a stratified sea

Abstract An island in a stratified region of the shelf seas creates a local increase in tidal mixing. The influences of the enhanced mixing on both the physical structure and phytoplankton biomass distribution have been assessed in a detailed survey of the Scilly Isles region of the Celtic Sea. Marked asymmetries in the observed pattern of stratification and sea surface temperature are in accord with the h/u3 distribution which predicts low stability regions occurring on the sides of the island. Displacement of the low stability regions relative to the h/u3 minima is consistent with a northward mean flow. Levels of biomass and primary productivity were found to be increased by a factor ∼5 over a large region (∼20 ×island area) surrounding the islands with particularly intense concentrations of phytoplankton in the pycnocline where chlorophyll levels up to 30 mg m−3 were observed. The location of these maxima at some distance from the islands is suggestive of an intrusive flow of mixed water into the pycnocline. Estimates of nitrate flux, associated with the production of mixed water by stirring, are of the right order to sustain the observed levels of production.

A modified manual method for the determination of urea in seawater using diacetylmonoxime reagent

A manual method for the quantitative determination of urea in seawater was developed based on the reaction of urea with diacetylmonoxime. Critical factors include the treatment of collection bottles, sample storage, pre-filtration techniques, water purification, reaction temperature and reproducible cooling of samples during analysis. The limit of detection was 0·14 μg-at urea-N 1 −1 with Beers Law being obeyed in the range tested of 0–15 μg-at urea-N 1 −1 . The precision (±1 SD) of replicate samples of 1,2 and 15 μg-at urea-N 1 -1 was 0·024, 0·019 and 0·03 μg-at urea-N 1 −1 ( n = 10) respectively.

Water column and sea-ice primary production during Austral spring in the Bellingshausen Sea

Abstract The findings of a cruise to study the phytoplankton bloom dynamics associated with the marginal ice zone (MIZ) in the Bellingshausen Sea during Austral spring (November-December) 1992 are reported. Biomass and rate process measurements were carried out at stations located in the ice, ice edge and open water along the 85°W meridian in order to establish the productivity of the microalgae associated with sea-ice and in the water column. In addition, a series of transects along 85°W from sea-ice to open water conditions enabled an assessment of the development of phytoplankton populations. Low phytoplankton biomass and production were noted at ice-covered and ice-edge stations and in the open water close to the ice edge. Observations from the transects indicated no development of a classical ice edge bloom despite evidence that sea-ice had retreated more than 100 km during the study period. Survey data along the 85°W line revealed a region of high chlorophyll, centred on 67.5°S, which was initially observed during brash ice conditions. This feature, which remained geographically consistent, persisted for at least 25 days and was thought to be associated with a frontal region. Water column primary production ( 14 C) in this high chlorophyll region was ca 0.8 g C m −2 day − , more than 8 times higher than noted in the MIZ. Phytoplankton photosynthetic characteristics within this region indicated that cells were adapted to a low light regime. A critical depth of 80 m, estimated directly from oxygen flux measurements, was sufficient to permit the initiation and net growth of phytoplankton standing stocks in a mixed layer of ca 70 m. A modelling approach using 14 C observations suggested that phytoplankton growth was less than the sum of the algal loss terms within this feature. An advective supply of cells therefore would be required to sustain the observed high and constant algal biomass. In addition, although this high chlorophyll feature was initially observed during brash-ice conditions, the available data suggest that it was initiated under open water conditions.

Size-Fractionated Primary Productivity In The Northeast Atlantic In May July 1989

Size-fractionated primary productivity measurements were done in the northeast Atlantic from May to July 1989 as part of the Joint Global Ocean Flux Study (JGOFS) North Atlantic Bloom Experiment. Samples were taken on the 20°W meridian at 47, 52, 56 and 60°N, with most intensive sampling at 47 and 60°N. At the time of the first sampling at 47°N, the bloom was over but, at the other stations, substantial phytoplankton biomass and productivity were measured. The northward progression in maximum phytoplankton biomass was associated with increasing water stability and resulted in a decline in the concentration of nitrate and silicate in the surface mixed layer. The maxima in phytoplankton biomass and productivity were associated with phytoplankton cells that were larger than 5 μm diameter, but the immediate pre-bloom conditions at 60°N were dominated by small nanoplankton ( 50% of the daily productivity of 500–800 mgC m−2 day−1. In the post-bloom conditions at 47°N, nanophytoplankton again became increasingly important. Picophytoplankton (<1 μm) accounted for a constant proportion (10–15%) of the total productivity. The implications of the measured productivity of the different phytoplankton size fractions are discussed in relation to vertical flux of organic carbon and the potential transport into deep water of carbon dioxide, which has been incorporated into particulate organic matter.

The BOFS 1990 spring bloom experiment: Temporal evolution and spatial variability of the hydrographic field

Abstract The overall background to the U.K. BOFS (Biogeochemical Ocean Flux Study) Project, designed to investigate oceanic carbon flux processes throughout the water column, is briefly described together with the strategy for the 1990 BOFS Spring Bloom Experiment. The Experiment involved two ships and was carried out in the northeast Atlantic between 46–50°N, 14–22°W in the period 18 April – 25 June 1990 with the objective of monitoring and quantifying the major carbon flux changes associated with the succession of the spring bloom. Sampling was carried out over a 7 week period adjacent to a Lagrangian buoy drogued at 30m. The spatial fields of the major variables were characterized from box grid surveys around the position of the marker drogue at the beginning and end of the time series observations with the time series hydrographical changes being related to features observed in the spatial surveys. The hydrographical and core biological observations made in the Experiment are described and interpreted. The reference drogue was deployed within an anticyclonic eddy in which initially there was little evidence of seasonal thermocline or phytoplankton develooment. The majority of an array of 30m drogues placed around the reference drogue drifted between 75–150km north and east of their origin, probably exiting from the original eddy system after the first 6 days of deployment. The reference drogue moved anticyclonically around the eddy centre for the first 13 days before exiting from the eddy system and becoming entrained in a discontinuity zone located between discrete warmer and cooler water bodies defined between 50–200m. During this latter period, which continued through to the end of the Experiment, the drogue tracked SE overall and alternately grazed the margins of the two water bodies with greater drift speeds being associated with the influence of the cooler water. Phytoplankton development proceeded slowly over the period that the drogue remained in the original eddy and paralleled the gradual development of the seasonal thermocline. A marked increase in phytoplankton occurred concurrently with the exit of the buoy from the eddy system into the boundary region of the cooler water where increased stratification prevailed. The phytoplankton increase persisted for only 6 days and declined sharply, primarily owing to advective influences, as the buoy moved away from the cool water influence with chlorophyll values remaining low for the remainder of the Experiment. Mesoscale influences were observed to have a major influence on the development sequence of the spring bloom in this area of the northeast Atlantic.

High dispersal potential has maintained long-term population stability in the North Atlantic copepod Calanus finmarchicus

The cool-water copepod Calanus finmarchicus is a key species in North Atlantic marine ecosystems since it represents an important food resource for the developmental stages of several fish of major economic value. Over the last 40 years, however, data from the Continuous Plankton Recorder survey have highlighted a 70 per cent reduction in C. finmarchicus biomass, coupled with a gradual northward shift in the speciess distribution, which have both been linked with climate change. To determine the potential for C. finmarchicus to track changes in habitat availability and maintain stable effective population sizes, we have assessed levels of gene flow and dispersal in current populations, as well as using a coalescent approach together with palaeodistribution modelling to elucidate the historical population demography of the species over previous changes in Earths climate. Our findings indicate high levels of dispersal and a constant effective population size over the period 359 000–566 000 BP and suggest that C. finmarchicus possesses the capacity to track changes in available habitat, a feature that may be of crucial importance to the speciess ability to cope with the current period of global climate change.

Seasonal variations in bio-optical properties of phytoplankton in the Arabian Sea

Data collected during three expeditions to the Arabian Sea are examined to characterise the changes in phytoplankton properties between the intermonsoon and monsoon seasons. The results are used to elucidate the key changes that lead to the occurrence of phytoplankton blooms following the onset of the monsoon winds. Contrary to expectations, changes in the assimilation number and the initial slope of the photosynthesis-light curve decreased in the SW monsoon season, apparently in the wrong sense for them to account for the incidence of blooms in the Arabian Sea during this season. On the other hand, there were strong differences between the intermonsoon and monsoon values of the chlorophyll-specific absorption coefficient for the phytoplankton, probably arising from seasonal changes in cell size and community composition. The sense of these changes was such as to increase substantially the maximum quantum yield for photosynthesis during the monsoon season compared with the intermonsoon. When these results are applied in a simple expression for the daily, primary production of the water column, they can explain the rapid accumulation of phytoplankton observed in the Arabian Sea following the start of the monsoon season. Thus, the occurrence of seasonal blooms in the Arabian Sea depends on combined variations in both the optical and photosynthetic characteristics of phytoplankton in the region.

Phytoplankton production and biomass estimates in the northeast Atlantic Ocean, May-June 1990

Time series measurements of size-fractionated chlorophyll a concentrations and primary production were made over the course of the spring bloom in and adjacent to a mesoscale eddy in the N.E. Atlantic as part of the U.K. Biogeochemical Ocean Flux Study 1990 Lagrangian Bloom Experiment. Substantial production, dominated by the >5μm and >1–5,μm fractions, was recorded before the main growth phase of the bloom, although total chlorophyll a concentrations varied little over this period. The main growth phase of the bloom, which occurred over approximately one week, was dominated by the > 1–5 μm fraction; a principal constituent was the small diatom Nanoneis hasleae, which almost entirely displaced larger diatoms more typical of a spring bloom population. The >5 um phytoplankton fraction became dominant for approximately two weeks following the main bloom growth phase but was replaced at the end of the observation period by the > 1–5 μm and 0.2-1 μm fractions. Phytoplankton taxonomic data for the 1990 bloom period were very different to the more typical bloom conditions of 1989. An intercomparison of14C production measurements made by U.K. and Dutch participants during the period of the Experiment using common samples gave total column productivities differing by <4%.