Paul Tett

The Transition Zone of the Canary Current Upwelling Region.

Abstract Like all the major upwelling regions, the Canary Current is characterised by intense mesoscale structure in the transition zone between the cool, nutrient-rich waters of the coastal upwelling regime and the warmer, oligotrophic waters of the open ocean. The Canary Island archipelago, which straddles the transition, introduces a second source of variability by perturbing the general southwestward flow of both ocean currents and Trade winds. The combined effects of the flow disturbance and the eddying and meandering of the boundary between upwelled and oceanic waters produce a complex pattern of regional variability. On the basis of historical data and a series of interdisciplinary field studies, the principal features of the region are described. These include a prominent upwelling filament originating near 28°N off the African coast, cyclonic and anti-cyclonic eddies downstream of the archipelago, and warm wake regions protected from the Trade winds by the high volcanic peaks of the islands. The filament is shown to be a recurrent feature, apparently arising from the interaction of a topographically trapped cyclonic eddy with the outer edge of the coastal upwelling zone. Its role in the transport and exchange of biogenic material, including fish larvae, is considered. Strong cyclonic eddies, observed throughout the year, drift slowly southwestward from Gran Canaria. One sampled in late summer was characterised by large vertical isopycnal displacements, apparent surface divergence and strong upwelling, producing a fourfold increase in chlorophyll concentrations over background values. Such intense eddies can be responsible for a major contribution to the vertical flux of nitrogen. The lee region of Gran Canaria is shown to be a location of strong pycnocline deformation resulting from Ekman pumping on the wind shear boundaries, which may contribute to the eddy formation process.

The influence of island-generated eddies on chlorophyll distribution: a study of mesoscale variation around Gran Canaria

This study reports hydrographic and biological observations from three cruises where cyclonic and anticyclonic eddies were observed downstream of Gran Canaria island. Based on field data and remote sensing images (AVHRR and CZCS), two mechanisms associated with island- generated eddies, largely responsible for the formation and distribution of chlorophyll around the Canary Islands, are proposed. First, nutrient pumping and vertical uplifting of the deep chlorophyll maximum by cyclonic eddies might represent important sources of primary production in the oligotrophic waters of the Canary region. Second, eddies are responsible for the horizontal transport and distribution of chlorophyll originating near the islands or off the African coast. Water with high chlorophyll content, resulting from island stirring or local upwelling at the flanks of the islands, is incorporated into cyclonic eddies in their development and subsequently transported downstream. On the other hand, anticyclonic eddies can also entrain water rich in chlorophyll when interacting with the offshore boundary of the African coastal upwelling. This chlorophyll will be advected southward as the eddy drifts. The recurrence of cyclonic and anticyclonic eddies, together with the presence of upwelling filaments throughout the year, must have important biological consequences in the formation and transport of organic matter in the Canary region. 0 1997 Elsevier Science Ltd. All rights reserved

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.

Problems in Modelling the Photosynthesis-Light Relationship for Phytoplankton

Is there a best model for the algal photosynthesis-light relationship? Jassby & Platts (1976) verdict favouring the hyperbolic tangent model is reassessed, the theoretical basis of model fitting is discussed, and a warning is given against equating model parameters with real physiological efficiencies or rates. To avoid bias models should be fitted without transforming data and parameters estimated simultaneously and independently. Use of such a method shows that the fit of several models cannot be distinguished from that of the hyperbolic tangent. One of these models is that of Smith (1936) and Tailing (1957 a), and the continued use of this model when there is no photoinhibition is commended on the grounds of relative computational simplicity and wide usage.

The Islay front: Physical structure and phytoplankton distribution

A shelf sea front exhibiting strong salinity influence is described. Temperature plays only a secondary role in controlling the density although the frontal position conforms to the h/u 3 model. Velocity measurements by current meters and radio drogues have defined the tidal flow and indicated residual currents of ∼20 cms −1 parallel to the front. These flows were time dependent with flow reversal occurring in the lower part of the water column. The observed velocity shear was of the same order as, but not in precise agreement with, the geostrophic shear inferred from the density gradient. In vivo and extracted chlorophyll measurements showed that phytoplankton standing crop at the front was several times greater, and was healthier, than that in the mixed water inshore. Chlorophyll was vertically stratified at the front and in the density-layered water offshore. High standing crop at the front may be explained in terms of nutrient availability to algae in the euphotic zone. The complex physical and biological processes influencing frontal productivity are discussed.

Modelling freshwater phytoplankton communities: an exercise in validation

Abstract A total of ten validation statistics were used to test a new freshwater phytoplankton model, PROTECH-C. The biological output of the model was compared with data collected from Blelham Tarn, in the English Lake District. The usefulness of the statistics and visual fits were evaluated, as well as PROTECH-C. It was concluded that only some of the statistics were suitable, given the magnitude and range of the data, and that visual fits are still vital, particularly with ecological data which can be episodic in nature. PROTECH-C was found to simulate total chlorophyll and the algal functional groups to a satisfactory level; its failure to model individual species was expected given the ecological level at which PROTECH-C simulates, i.e. species’ populations.

The General Annual Cycle of Chlorophyll Standing Crop in Loch Creran

(1) This paper introduces an investigation of the factors controlling phytoplankton growth in Creran, a typical sea-loch on the west coast of Scotland. (2) The hydrography of the loch is summarized. Salinity, irradiance and photosynthesis profiles suggest that the lower limits of the reduced-salinity surface layer and of the euphotic zone can conveniently be taken to coincide at a depth of 8 m. (3) The general annual cycle of phytoplankton standing crop in the loch is shown by a combined plot of 908 chlorophyll a measurements made between 1970 and 1976 in the 1-8 m depth zone. Limits that theoretically include 95% of the data are superimposed on this plot, and show that a number of features recur regularly from year to year. (4) These recurring features are: a winter low ( < 0 5 mg chl. m 3); a spring increase, with chlorophyll reaching 1 mg m -3 at some time between 17 February and 12 March, and attaining 12-37 mg chl. m 3 between 13 March and 11 April; a biomass of 1-7 mg chl. m3 from June until mid-September; no definite autumn bloom; and a reduction of chlorophyll levels to below 1 mg m3 by 26 November. (5) The importance of salinity layering in the control of the phytoplankton cycle is briefly discussed. The spring increase in Creran starts at about the same time as in other sea-lochs and fjords, and thus 1 or 2 months earlier than in similar seas where the stability of the water column is controlled by temperature. (6) The theoretical basis of the 95% limits superimposed on the data plot is discussed in an Appendix. It is suggested that these limits can be used predictively.

Biological consequences of tidal stirring gradients in the North Sea

Tidal stirring gradients, interacting with seasonal variation in photosynthetically active radiation, sea-surface heating, and wind stirring, are proposed as the most important controls on plankton in the southern North Sea. The hypothesis, in the form of a numerical model, is tested against observations during 1988/89 of seasonal cycles and spatial variation in phyto- and zoo-plankton. The importance of the tidal mixing front, and the effects of residual circulation and nutrient-rich river discharges, are discussed, and estimates given of microplankton community production and its fate.

A model for the growth of shelf-sea phytoplankton in summer

Abstract The model describes the steady-state vertical distribution of phytoplankton chlorophyll biomass in a shelf sea in summer as a function of vertical turbulent diffusion, surface photosynthetically effective irradiance, the diffuse attenuation of this irradiance, grazing pressure, and deep-water nutrient flux. It includes equations for nutrient uptake and nutrient-controlled growth, net photosynthesis, and the recycling of nutrient by grazers. It differs from related ‘vertical-process’ models in using cell-quota, threshold limitations, theory for nutrient- and light-limited algal growth. The model was used to predict the distribution of chlorophyll on a section near the Scilly Isles, England, in July 1979, and in the Sound of Jura, Scotland, in July 1983. The former is a mainly stratified region into which is intruded island mixing; the latter is a predominantly mixed strait with a small stratified area. Nitrogen was taken as the limiting nutrient, and the values of most parameters were obtained either from the literature or from independent observations. The models predictions were particularly sensitive to values chosen for photosynthetic fixation per unit irradiance at low illuminations, nutrient subsistence quota, and the chlorophyll-carbon ratio. Prediction was unsuccessful in the case of the Sound of Jura, probably because of the short time taken by residual flows to cross the stratified region. Results for the Scilly Isles show, however, that despite uncertainties in the parameterization of diffusivity, grazing and benthic nutrient flux, this simple, vertical-diffusion driven, model predicts a substantial part of the distribution of phytoplankton in a typical shelf sea in summer, and can clarify the processes determining the growth and production of planktonic algae. Minimal estimates of primary production near the Scilly Isles in summer range from about 160 mg C m −2 day −1 in well-stratified water columns to about 700 mg C m −2 day −1 in frontal conditions resulting from island mixing.

Observations and simulations of hydrography, nutrients and plankton in the southern North Sea

Abstract Much of the southern North Sea is shallow. Strong tidal stirring opposes stratification, and increases turbidity through sediment resuspension. Consequently, phytoplankton growth is often lightcontrolled in this region, although riverine nutrients increase production and maximum biomass in waters near the continental coast, where Phaeocystis and small copepods are common. In deeper waters to the north, seasonal stratification gives rise to a ‘text-book’ annual cycle of plankton, with the spring bloom followed by a period of severe nutrient depletion. These points are illustrated with results from repeated surveys of the southern North Sea in 1988–89, and further examined using a simple mathematical model. Observations and numerical simulations are used to provide insights into nitrogen cycling and eutrophication potential, which are greatest in regions of intermediate tidal stirring, and to support a discussion of what is needed by models if they are accurately to simulate nutrient cycling.