(Contract MAS2-CT930054)
EC MAST project PROFILE (contract MAS2-CT930054) studies the role of physical processes controlling water-property distributions, suspended sediments controlling the availability of light, nutrients and phytoplankton growth. The project comprises development of a 3-D nearshore model, measurements in contrasted ROFIs and comparisons between them.
3-D nearshore model, with high resolution (~ 1 hour, 1 km).
Systematic measurements in contrasted ROFIs, suitable for testing the nearshore model (time- series over a seasonal cycle for dynamics and intermittency, good vertical resolution and spatial surveys measuring stratification, SPM and plankton.
The Rhine gives a large, tidally-pulsed discharge from a narrow waterway, at a straight coast where strong tidal currents and wind forcing are often sufficient to mix the coastal waters vertically through depth, typically 20 m in the outflow plume. When winds are weak, however, the fresher inshore water tends to flow offshore by gravity over the denser offshore water, giving rapid re- stratification, particularly at neap tides. The plume typically extends along the coast north- eastwards and 30 km offshore. In 1990 and 1992, measurements were made in summer and autumn; in 1994, from March to October, with emphasis on the spring bloom in April-May. The 1994 experiments comprised: mooring time-series; repeat sections across the plume; spatial surveys and water sampling.
An unusual bloom occurred in August 1994; summer heating and continual stabilisation by the near-surface salinity deficit caused an exceptionally warm upper layer. In April 1994, the following sequence of events has been inferred: strong wind-mixing, hence available nutrients near-surface and an upper-layer bloom; less mixing, death and settling of the bloom, pycnocline formation; trapping of the sunken phytoplankton (higher SPM concentrations) below the pycnocline; on- offshore tidal straining (associated with stratification) alternately enhancing and removing the pycnocline, allowing phytoplankton to mix up towards the surface at the latter times. Another period of strong mixing would re-start this sequence. Modelling of the Rhine outflow has simulated many observed features: development of the plume through tidal cycles of pulsed outflow; formation of a front on the (tidally) upstream side; the plume's first anticyclonic inertial turn; reduction of plume spreading and stratification, by wind-mixing and stronger tides; offshore spreading and retardation of the plume by an "upwelling-favourable" wind; acceleration and nearshore confinement of the plume by "downwelling-favourable" wind. A 1-D (vertical) model for turbulence and SPM has successfully simulated time-series of the vertical distribution of SPM.
The Clyde Sea is fjordic, relatively enclosed with basins of maximum depth exceeding 180 m and a sill of depth about 50 m at the southern end. Weak tidal currents and continuous river input lead to almost permanent stratification. A long residence time for the bottom waters also attaches interest to their nutrient regeneration and benthic uptake of oxygen. A seasonal cycle of measurements spanned March 1993 to May 1994 at monthly intervals approximately. Measurements comprised CTD profiles, sampling and two time-series moorings.
Mixing through depth occurred just once, in late November 1993. There was a near-surface seasonal heating/cooling cycle. A gradual rise in temperature and decrease in salinity of deep water through the summer, and other evidence, indicated admixture of near-surface water, and has been successfully simulated by a "filling-box" model. Chlorophyll increased early (March) in the presence of stratification over the Arran Deep, whilst reinforcement of stratification by surface warming through May and June promoted the spread of high algal biomass to the Great Plateau. There were corresponding variations in nutrient depletion. From May onwards, there were increases in nutrient concentrations in the basins, associated with remineralisation of organic matter in the isolated deep water and sediments. The essential features of these seasonal cycles have been investigated with models. Sediment oxygen uptake was measured as 32-36 mmoles per square metre per day. Repeat transects in March 1995, at the season when deep renewal by cross-sill exchange has been evident, found a net inflow in the east and a net outflow from the western side of the Clyde Sea.
Thermaikos Bay is a shallow gulf of breadth 10-20 km, with weak tides but very strong seasonal signals in river run-off, heating and cooling, and variable wind forcing. "Red tides" have been recorded in the inner Bay and earlier data show increasing nutrient loads. Measurements for a year (spring 1994 to summer 1995) comprise two time-series moorings, monthly CTD surveys and monthly or more frequent biological surveys.
In general, more chlorophyll was found at the head of the gulf, and in the euphotic zone from January (1995). Temperature ranged from 13 C in March to 27 C at the surface in August, with a maximum (summer) top-bottom difference 9 C. In summer-autumn, intrusions of cooler water from the Aegean were observed for short periods (a few days each). Autumnal cooling reduced the surface temperature below that of water underneath, at the southerly mooring site, then stabilised by location in the plume with fresher surface water (usual except in January). Freshening in the plume was strongly correlated with run-off. 3D modelling for tides and an average seasonal cycle of heating, rainfall, river inflows and winds has shown an anti-clockwise circulation in winter, as known to occur under northerly winds. Southerly winds give flow into the Bay in the middle of the entrance. Calculations with no riverine inputs show reduced circulation and increased flushing times, which are also strongly affected by winds.
The German Bight is an extensive shallow area; strong tidal mixing, topography and the locations of freshwater sources control the salinity distribution. The German national research project PRISMA took place from 1990 to 1993, investigating physical and biological processes controlling contaminant fluxes, by means of field measurements, remote sensing and model simulations. A 2.5km-resolution 3-D baroclinic model of the German Bight has been developed, prognostic for currents, temperature and salinity; driving is by surface winds and heating, freshwater discharge from the river Elbe and open-sea boundary conditions from a model of the whole north-west European shelf. The model has been validated against measurements in PRISMA, and used to budget contaminant fluxes and to estimate Reynolds stresses and turbulent exchange of buoyancy. As a follow-up, the project KUSTOS was established, aiming to quantify energy and nutrient fluxes in the German Bight. KUSTOS is a national contribution to IGBP-LOICZ.
The Po discharges a large quantity of freshwater seasonally into the northern end of the Adriatic, typically with a maximum in river material fluxes in late spring or early autumn. The ROFI extends variably eastwards across the northern Adriatic, especially in summer, and south-eastwards along the Italian coast, especially in winter. Data collected prior to PROFILE include: daily run-off, fortnightly nutrients, suspended matter and oxygen therein; four CTD surveys of the northern Adriatic in 1993 under various rates of run-off; regular 1993 sections offshore from the Italian coast south of the outflow. A 3-D finite-element model has been developed for the Adriatic with finer resolution near the Po mouth and the Italian coast. A reference run, forced only by a Po discharge 1000 cubic metres per second, showed the outflow spreading along the Italian coast to the north and south. With 5000 cubic metres per second discharge, the outflow extended nearly twice as far in every direction. With 5 m/s ("downwelling-favourable" bora) wind from the north-east, the outflow spreads only to the south, confined against the coast.
Comparison of Regimes. The studies of contrasted ROFIs are being synthesised by comparing them as systems with respective:
Product. Via the EC-MAST supporting initiative EDAP, project observations, model code and output are being prepared for distribution (CD-ROM) as test data and a nearshore model for wider use.