Abstract:
Production rates for natural populations of bacteria and diatoms epiphytic on macroalgae were determined for selected hosts at sites in the SW Baltic, West Germany and within the Hauraki Gulf, New Zealand. Based on tracer uptake patterns t relative production rates for epiphytic bacteria were determined both as heterotrophic activity, using 14C glucose as substrate, and reproductive activity, using H thymidine. Results based on both of these parameters indicated that macroalgae with fine thalli support greater bacterial activity per gram dry weight than do those with coarse form. Per unit surface area interspecific differences were less marked and no clear pattern based on the form of the host was evident. Intraspecific variability was apparent from the monthly records and was confirmed by microautoradiograph and SEM studies. The bacterial heterotrophic activity associated with most hosts from the Hauraki Gulf was lower than that for hosts of equivalent form from the SW Baltic. Seasonal differences were noted especially at the Baltic sites where temperature differences are marked. Epiphytic diatom production rates were assessed by measurement of photosynthesis. A method of separating some epiphytic diatoms from their host employing proteolytic enzymes was devised which when used in combination with conventional 14C bicarbonate, 02 Winkler, O2 electrode or modified C02 IRGA techniques allowed photosynthetic activity of the two components to be determined separately. Epiphytic diatoms that were able to be removed by simple wiping or forcep action were also available for photosynthetic activity determination. Rates of photosynthetic activity for diatoms were recorded either as g C mg Chi a-1 h"1 or as mg C cm-3 h"1. Assimilation numbers ranged from approximately 1.5 to 4.5 mg C mg Chi a~l h_1 while values based on volume -3 -1 ranged from approximately mg C cm h . The values were based on only the limited number of species of epiphytic diatoms that could be removed from their host. However, the volume density for each of the diatom species associated with each host was able to be calculated from enumeration of acid cleaned frustules and such values were used, by extrapolation of the limited volume based carbon fixation data available to obtain an approximate total epiphytic diatom photosynthetic activity. On this basis most of the epiphytic diatom assemblages were shown to fix carbon at a mean rate of between 1 and 10 mg C m (host)-2 h-1 r between 0.1 and 3 mg C g dw(host)-1. Comparing thse values with host photosynthetic rates it is concluded that diatoms contribute between 10 and 50% of the total photosynthetic activity for most fine thallus host-epiphyte assemblages whereas for coarse hosts the contribution is in most cases less than 10%. Diatom volume density records indicate that most macroalgae with fine thalli support greater diatom activity per gram dry weight than those of coarse form. On a surface area basis, however, interspecific density differences are not so marked and no clear pattern, based on the form of the host, was evident. Seasonal differences in volume density were evident, especially at the Baltic sites where seasonal temperature differences are marked and where many of the hosts are annual plants with a relatively short growing season.