Responses of continuous-series estuarine microecosystems to point-source input variations.

A group of six continuous-series microecosystems were constructed to stimulate hydrological factors of estuarine regions. Exchange and retention characteristics were adjusted to closely model the hydrological conditions of Trinity Bay, Texas. The responses of the microecosystems to quantitative and qualitative changes in freshwater input were investigated. Due to fundamental similarities of all living systems, these responses might be especially representative of those which would occur in Trinity Bay if subjected to similar conditions; actual testing of this conjecture was logistically and operationally infeasible. Magnitudes of primary production and community respiration in the freshwater portions of the microecosystems were dependent on both quantity and quality of freshwater input. Magnitudes of primary production and community respiration in the saltwater portions of the microecosystems were virtually independent of the quantity and quality of freshwater input. The metabolism of the freshwater portions of the microecosystems was heterotrophic under conditions of 60 day freshwater retention time and autotrophic under conditions of infinite freshwater retention time. Addition of an industrial effluent to the freshwater inputs resulted in extensive shifts towards metabolic heterotrophy by the more freshwater portions of the microecosystems. The metabolism of the saltwater cell positions was heterotrophic under all conditions of freshwater input. The communities of the freshwater portions of the microecosystems adapted to a dependency on allochthonous materials input for production and respiratory maintenance. The retardation of freshwater input resulted in larger portions of the nutrient pool within the systems being tied up in living components. The addition of industrial effluent increased the maintenance requirements of the communities. The retardation of freshwater input acted as an environmental stress on the freshwater portions of the microecosystems. Magnitudes of production and respiration were significantly lower, and zooplankton standing crops and species diversity decreased significantly. The addition of industrial effluent produced the same effects. There was a synergistic effect of reduced freshwater input and effluent loading on the freshwater portions of the microecosystems when the stresses were applied simultaneously. The effect of one of the stresses on the receiving systems rendered the communities more susceptible to the imposition of the secondary stress. The organismal composition of the microecosystems was qualitatively similar but quantitatively dissimilar to the organismal composition of Trinity Bay. Periphytonic flora gradually attained the dominant producer status, and were the successional aftermath of planktonic floral dominance. Additional studies indicated that: low level grazing by herbivores enhanced the primary productivity of the periphytonic flora; CO2 enrichment enhanced primary production only in the freshwater portions of the microecosystems; and rates of production, respiration, and zooplankton standing crop magnitudes were largely independent of long term acclimation temperatures.