Biological studies in the vicinity of a shallow-sea tidal mixing front VI. A general statistical study

Abstract

Studies of the distributional properties, the interrelationships and comparisons in time and space of 21 biological, chemical and physical variables that characterize the activities in a shallow sea tidal mixing front in the Western Irish sea are presented. They represent an attempt at describing and interpreting biologically this complex ecosystem as a whole and particularly to assess and compare the intensity of biological and biochemical activities and differences in distribution of organisms between the upper and lower stratified and mixed water columns on the two sides of the front at different times of the year. The analyses used were mainly parametric methods, but non-parametric analyses were found to be appropriate in a few cases. The log-normal distribution tended to fit better than the normal for most variables for each water mass within each cruise. Also different discrete distributions were fitted by the method of maximum likelihood to the bacterioplankton and zooplankton data and the best fits in both cases and for each cruise, where adequately large data was available, turned out to be the negative binomial distribution. Some of the associations between the variables for separate water masses in each cruise, described by the non-parametric Spearman rank correlations, had meaningful biological interpretations while others did not. Also structural simplification through reducing the dimensionality (15 variables) of the system produced, by using principal component analysis on logarithmically transformed data, a few components that persisted throughout the cruises in the upper stratified water and could be interpreted in ecological terms; notably components showing the effect of physical stratification on biological activity, the depletion of nitrogenous compounds by plankton and the possible effect of protozooplankton grazing. Comparisons of the levels of biological and biochemical activities determined by parametric, with data logarithmically transformed, and non-parametric one-way analyses of variance (ANOVA) showed significant differences between these levels particularly glucose and urea uptake rates, consistently in the three water masses and in all cruises. The dominant feature was that the upper stratified water was different from the other two water bodies and that the two methods of analysis produced similar results. The relative importance of the biological variables to differentiate the water masses was assessed by using stepwise discriminant analysis, on data transformed logarithmically, and this confirmed, to a large extent, the results obtained from the analysis of variance comparisons. Differences between the three lifestages of zooplankton numbers were ascertained by using a randomized block ANOVA on logarithmically transformed data which indicated that these differences were significant in all but one of the eight cruises where data was available. Significantly greater abundance of zooplankton haul numbers were found at the front and the stratified side compared with the mixed side. The diurnal variation of zooplankton numbers modelled by multiple regression analysis with data again transformed logarithmically showed that the numbers depended on depth in the stratified water column but on time in the mixed water column. The analyses overall showed that the upper stratified water is an area of intense biological activity especially in the vicinity of the front and on the whole has many different characteristics from the rest of the water body and that most variables are closely linked, particularly during stable stratification in the summer.