The Movements of a Marine Copepod in a Tidal Lagoon
It has been hypothesized that marine copepods are able to react to water movements in an estuary in such a way as to minimize ad-vective losses to the open ocean. This hypothesis is developed mathematically and tested by use of a stochastic model. Several processes are investigated as contributing to the minimization of advective loss.
The model treats time-varying spatial patterns of marine plankton. In the present instance, the model is used to describe the movements of Acartia tonsa (Copepoda) in a tidal lagoon on the eastern end of Galveston Island, Texas. Four distinct processes are considered: advection by currents, behavioral response to environmental variables (current velocity, temperature, and salinity fields), intraspecific aggregation, and birth-death processes. The portion of the model dealing with biological processes is a stochastic compartmental model. The biological model is driven by a three-dimensional physical dynamic model which provides numerical solutions for current velocity, temperature, and salinity fields.
The coupled physical-biological model used to simulate the distribution of A. tonsa provided numerically accurate estimates for the time histories of the physical and biological processes involved. The success of the model was probably attributable to a number of factors which involve the nature of the ecological situation modeled, the form of the parameters in the biological model, the manner in which the lagoon was compartmentalized, and the nature of the sampling data used to test the results.
Assuming then that the results of the numerical simulation were accurate by other than random chance, the most important conclusion was that A. tonsa appears to owe its spatial distribution in the lagoon to the combined effects of advection by currents and behavioral response to environmental stimuli: tides, light, temperature gradients, salinity gradients, and the population density gradients of its own species; the most important being tidal advection. It also appears that the resultant movements of the organism are sufficient to minimize losses from the lagoon to the extent that it maintains an endemic population inside the lagoon which is distinct from the population found immediately outside. Finally, it was concluded that the spatial distribution of A. tonsa is heterogenous, that the patches are of the order of 240 metres long by one or two metres deep, and that changes in density occur as a result of an increase in within-patch density rather than an increase in the number of patches.
KeywordsCurrent Velocity Salinity Gradient Biological Model Water Surface Elevation Salinity Field
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