Ken Jones

Evidence of in situ diel vertical migration of a red-tide microplankton species in Ria de Vigo (NW Spain)

The migration capacity of red-tide species in the natural environment was studied at a station in the Ría de Vigo (Rías Bajas, NW Spain) over a 24 h period in September 1991. The Ría de Vigo, where red tides are frequent, normally shows a positive estuarine circulation and is subjected to seasonal upwelling and downwelling phenomena. A marked diel pattern was observed for five species that are capable of causing red tides (Ceratium furca, Scrippsiella trochoidea, Dinophysis acuminata, Mesodinium rubrum, and Eutreptiella sp.). Such diel behaviour could be clearly advantageous in a stratified environment where light and nutrients are often in two separate layers. Active movement enables species such as dinoflagellates and some ciliates to exploit high levels of irradiance at the surface during the day and to take up nutrients in deeper layers at night. Patchy distribution of phytoplankton, shear-induced horizontal dispersion and density variations were considered, but none of them accounted for the vertical changes observed. Vertical migration is thought to be one of the mechanisms that could promote blooms in nutrient-depleted surface layers.

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.

Influence of stratification and irradiance regime on summer phytoplankton composition in coastal and shelf seas of the British Isles

Abstract The effect of interaction between stratification and irradiance regimes on phytoplankton community structure was investigated in three shelf/coastal regions of the British Isles, each of which displayed ranges of vertical stability and light attenuation. Relationships between vertical stability, light penetration and community structure were indicated by the ordination of dominance (assessed using cell volume) of the main phytoplankton phyletic groups—diatoms, dinoflagellates and microflagellates—on a surface defined by a bulk stratification index (ϕ) and water column depth, scaled by transparency (λ h ). Diatom- and dinoflagellate-dominated communities occupied distinctly different domains on the ϕ - λ h surface, diatoms being favoured in well-mixed water columns with high values for λ h and dinoflagellates dominating where stratification was strong and λ h was low. Microflagellates were not abundant in any of the study areas and showed no clear ordination on the ϕ - λ h surface. The domain of co-dominance of diatom and dinoflagellates on the ϕ - λ h surface was narrow with small changes in the irradiance or stratification regime resulting in a switch to diatom or dinoflagellate dominance. It is suggested that loss of non-motile diatom communities in strongly stratified water columns might be a strong selective force in favour of dinoflagel-lates. However, in water columns with intermediate stratification and optical properties, the outcome of competition may be decided by physiological attributes of the two groups with respect to growth in low and fluctuating irradiances.

The PROWQM physical-biological model with benthic-pelagic coupling applied to the northern North Sea

PROWQM, a 1-D depth resolving model which couples physical and microbiological processes in the water column with sedimentation/resuspension and benthic mineralisation processes, has been used to simulate seasonal changes of chlorophyll, nutrients and oxygen at the PROVESS north site (59degrees20 N 1 degrees00 E) in the North Sea. PROWQM is derived from the 3-D model COHERENS, and improves COHERENs benthic and pelagic biology. The physical sub-model of PROWQM implicitly solves turbulence closure equations forced by climatological, or realistic high-frequency, meteorological and tidal data. The pelagic biological sub-model 2MPPD includes a diatomy microplankton (mp1) and a flagellatey (or microbial loop) microplankton (mp2), the cycling of silicon and nitrogen, slow-sinking detritus, and fast-sinking phytodetritus. Phytodetritus is formed by shear-driven aggregation of particulate material, using a simple algorithm. for bulk processes that is derived by considering the interactions of single cells. The microplankton compartments include heterotrophic bacteria and protozoa as well as phytoplankton, and most microplankton rates are specified with the aid of a heterotroph fraction parameter, which was 0.125 for mp1 and 0.6 for mp2. The microbiological system is closed by mesozooplankton grazing pressures imposed as time varying series determined from observed zooplankton abundance. The benthic boundary sub-model includes a superficial fluff layer and a nutrient element reservoir in the consolidated sediment. Particulate material in the fluff layer can be resuspended (in response to bed stress by near-bed flows), mineralised or carried by bioturbation into the underlying, consolidated, sediment, where it is mineralised and its nutrients returned to the water-column at rates mainly dependent on (implicit) macrobenthic pumping. Benthic denitrification can occur when mineralisation rates exceed oxygen supply. Verification of the PROWQM numerical implementation used test cases and checks for nutrient element conservation. Simulations with realistic forcing, for a range of parameter values, were compared with historic observations in the NOWESP data set and during FLEX76, and with those made during the PROVESS cruises in autumn 1998. PROWQM provided a good simulation of the seasonal succession from a diatom-dominated spring bloom to summer dominance by small flagellates. The simulations included sedimentation of organic matter from the spring bloom, and qualitatively realistic behaviour of the fluff layer, but decay rates were too slow and there was almost no denitrification. The simulated surface mixed layer was too shallow during,the summer. Simulated annual net microplankton primary production was in between 59 and 91 g C m(-2) y(-1). A large proportion of mineralisation, 28-47% of nitrogen and 40-67% of silicon mineralisation, took place as a result of the decay of sinking and resuspended detritus whilst in the water column. PROWQM is discussed in relation to other models that have been used to simulate this part of the North Sea, in particular the simpler ECOHAM1 and the more complex ERSEM, and in relation to PROWQMs, evolution from COHERENS

Steady-state mixing of iodine in shelf seas off the British Isles

Abstract A study of dissolved iodate and total iodine on the Hebridean shelf in several months of 1995/6 confirms an earlier hypothesis that the distribution of iodine in these waters is almost in steady state throughout the year. No evidence was found for changes in iodine chemistry that might correlate with seasonal changes in primary productivity, and the inappropriateness of adopting a Redfield-type model to explain the main trend of the iodine distribution is discussed. The system is determined by the mixing of three water types. To the west, the iodine distribution is governed by water of the North Atlantic Current (typically with salinity of 35.3 and iodate and total iodine concentrations close to 0.40 and 0.45xa0μM, respectively, and its mixing with deeper water containing a slightly higher concentration of iodate, but similar salinity. To the east, the distribution is governed by coastal waters with low salinity (∼33) and low iodate concentration (0.25xa0μM) but only a marginally lower total iodine concentration. The approximate iodate concentration, rationalised to 35 salinity, is given by RIO3=0.060e−22 (35.42−S)+0.37e−0.20 (35.42−S)+0.010e−0.010 (35.42−S). Rationalised total iodine varied by only about 5%. Reasons are given for believing that the conditions found in the Hebridean waters probably applies to the whole of the shelf-seas of Britain, including the North Sea.

The phytoplankton ecology of the Firth of Clyde sea-lochs Striven and Fyne

Lochs Fyne and Striven are deep-silled fjords opening into the northern Firth of Clyde. They receive relatively little freshwater from local runoff, have a relatively low rate of tidal exchange, and undergo periods of deep water stagnation. Both contain fish farms which have lost stock because of phytoplankton blooms. Loch Striven was investigated in detail in 1980. Observations are also reported for 1979 and 1981, and for Loch Fyne from 1980–1982. Most stratification in Striven is due to salinity layering, but most of the freshwater in the loch probably originates in the Clyde Estuary and is associated with high concentrations of nitrate. Typical phytoplankton biomasses in Striven are the same as those in the western seaboard Loch Creran, but maxima exceed those in Creran. In addition to a red tide of Gyrodinium aureolum in September, 1980, three periods of high biomass seem to be a regular feature of the phytoplankton calendar in Striven. The spring increase takes place in March or early April, is dominated by the diatom Skeletonema costalum, and probably depends on the stabilizing effects of reduced near-surface salinities. A summer flourishing of Leptocylindrus danicus and dinoflagellates is also associated with near-surface salinity layering. A late spring bloom of diatoms, dinoflagellates and small flagellates, some ichthyotoxic, appears to be related to the occurrence of thermohaline stratification and near-surface nutrient depletion; on some occasions lochhead upwelling may also have been involved. Insofar as data are available, the ecology of phytoplankton in Loch Fyne appears similar to that in Loch Striven.

The hydrography and phytoplankton ecology of Loch Ardbhair: A small sea-loch on the west coast of Scotland

Abstract The hydrography and phytoplankton ecology of Loch Ardbhair, a small sea-loch on the northwestern coast of Scotland, was studied between January 1981 and July 1982. Tidal exchange was the dominant hydrographie feature of the loch and the exchange time was estimated to be between 1 and 2 days. In 1981 seasonal changes in phytoplankton biomass and species succession were similar to other sea-lochs, although the spring peak, and summer levels of, phytoplankton biomass (3.0 and 1.0 mg chlorophyll · m−3 respectively) were lower than might have been predicted from winter levels of nitrate. Chlorophyll levels in Ardbhair were much higher in July 1982 (up to 3.5 mg · m−3) compared to July 1981 (0.7 mg · m−3) and the phytoplankton was dominated by dinoflagellates rather than microflagellates and diatoms. It is suggested that rapid exchange limits in situ growth of phytoplankton in Ardbhair and that phytoplankton biomass and species composition in Ardbhair closely resembles that of Eddrachillis Bay with which the loch exchanges. The importance of these results concerning the risk, to loch-based fish farms, from ichthyotoxic blooms is discussed.

Water column structure and summer phytoplankton distribution in the Sound of Jura, Scotland

Abstract The distributions of water column stratification, phytoplankton and nutrients were investigated in the Sound of Jura, Scotland, during July 1982 and June–July 1983. The amount of stratification was mainly related to variation in tidal currents and bathymetry along the length of the Sound. Frontal zones separated the well-mixed northern Sound, and lateral regions, from thermally stratified water in the middle of the Sound. In the south transitionally stratified water extended into the North Channel of the Irish Sea. The phytoplankton in well-mixed regions was dominated by diatoms, whereas naked dinoflagellates and microflagellates were important in stratified regions. The distribution of phytoplankton biomass, indicated by chlorophyll concentration averaged over the water column, is explained by the effects of vertical mixing on the average illumination experienced by phytoplankton and by the supply of nutrients to the euphotic zone. Highest mean water column chlorophyll was found near Islay, perhaps due to special local input of nutrients into the euphotic zone there. Biomass was also high in the southern region of transitional stratification. The northern and lateral fronts were not clearly associated with high phytoplankton biomass, perhaps because of the small size of the features in relation to the advective field. Phytoplankton biomass was generally low in well-mixed waters, and at the surface in well-stratified regions. It is suggested that transitional stratification may enhance productivity in the southern region to two to three times that in the well stratified and mixed regions of the Sound. The variations in stratification along the Sound may have important implications for primary productivity in the lochs that exchange with it.

Inherent and apparent optical properties in coastal waters: a study of the Clyde Sea in early summer

Profiles of absorption and attenuation coefficients, downward irradiance and upward radiance were measured at 14 stations in the Clyde Sea during May 2000. The absorption coefficient at 676 nm was linearly correlated with chlorophyll concentration (R2=0.88), and the ratio of scattering to absorption in this waveband could be used to discriminate between phytoplankton and other suspended material. The radiance and irradiance measurements showed rapid attenuation of blue and red wavelengths near the surface, while the red component was augmented by chlorophyll fluorescence in deeper waters. Throughout the Clyde Sea the relationship between absorption coefficients a(λ), scattering coefficients b(λ), and diffuse attenuation coefficients for downward irradiance Kd(λ) was well described by a function proposed by Kirk (Limnol. Oceanogr. 29 (1984) 350). A procedure was devised for retrieving inherent optical properties from measurements of the radiance reflectance and diffuse attenuation coefficient that was able to successfully predict measured absorption coefficients throughout the study area (R2=0.95). Backscattering ratios estimated using this procedure varied with depth and location according to changes in the relative concentration of phytoplankton to other particles. Tables of measured inherent and apparent optical properties and derived backscattering ratios for near-surface water are provided for modelling radiative transfer in the Clyde Sea and other coastal areas subject to freshwater influence.

The seasonal cycle of nitrate in the Clyde Sea

Abstract The Clyde Sea is a deep, partially enclosed basin on the West Coast of Scotland which communicates with the adjacent North Channel of the Irish Sea via exchange flow over a relatively shallow entrance sill. The basin receives large inputs of nutrient rich freshwater which induces a reduction in surface salinities of up to 1.5 psu and maintains almost continuous stratification. Time series data collected in the area show a well defined seasonal cycle of stratification which has previously been explained with the aid of a one-dimensional filling box model. This model has been extended to examine the extent to which the observed seasonal cycles of nitrate and chlorophyll can be explained by the stepwise inclusion of additional non-conservative terms. The results show that, as a result of the interaction between the physical and biological seasonal cycles, nitrate accumulates in the Clyde Sea during the summer, which is then supplied to the North Channel of the Irish Sea during the winter and spring.