Screening, biogeochemical, and ecological, models of enriched marine coastal ecosystems

Paul Tett
Dept. of Biological Sciences
Napier University
Merchiston Campus
10 Colinton Rd.,
Edinburgh
EH10 5DT

Eutrophication is defined by the EC as:

'enrichment of water by nutrients especially compounds of nitrogen and phosphorus, causing an accelerated growth of algae and higher forms of plant life to produce an undesirable disturbance to the balance of organisms and the quality of the water concerned.'

Implicit in the definition, the history of the concept, and the use of the word 'undesirable', is the idea that eutrophication is a process and that it is a matter of concern when this process results substantially from human actions and changes marine ecosystems from their 'natural' state. The definition may be broken down into 3 causally-linked stages, all of which must occur for eutrophication to be recognised. Objective standards have been proposed for recognition of the first and second stages, but not as yet for stage 3. My aim in this talk is to consider the use of several types of model for diagnosing or predicting each stage in the eutrophication process. I will show that stages 1 and 2 are relatively easy to deal with, whereas stage 3 is difficult: it is at the limit of our current ecological modelling capability.

The models considered are: (1) a screening model based on the recommendations of the UK Comprehensive Studies Task Team, in which the key terms are bulk exchange rate and the yield of chlorophyll from nutrient; (2) a series of dynamic models of increasing complexity, culminating in the 2-microplankton model 2MMPD. Microplankton models jointly parameterise pelagic autotrophic and heterotrophic processes: i.e. they include microalgae, bacteria and protozoa in the same compartment. Mesozooplankton activity is represented by a grazing pressure applied from observed zooplankton abundance, often taken from CPR data. Results from 2MPPD are shown for the northern North Sea and the Firth of Clyde using different physical models. Results are shown from numerical experiments concerning the effect of changing N:Si ratios and of removing protozoan grazing.