Responses of Continuous‐Series Estuarine Microecosystems to Point‐Source Input Variations

Abstract

Six continuous—series microecosystems, each containing five cells, were constructed to simulate hydrological factors of estuarine regions. Exchange and retention characteristics were adjusted to closely model the hydrological conditions of Trinity Bay, Texas. The metabolic and structural responses of the microecosystem communities to quantitative and qualitative changes in freshwater input were investigated. Primary production and community respiration in the first three cells of the microecosystems were dependent on both quantity and quality of freshwater input, whereas primary production and community respiration in the saltwater portions (cells 4, 5) of the microecosystems were virtually independent of the quantity and quality of freshwater input. Metabolism of the freshwater portions of the microecosystems was heterotrophic under normal flow conditions and autotrophic under drought conditions. Addition of an industrial effluent to the freshwater inputs resulted in extensive shifts towards metabolic heterotrophy of the more freshwater portions of the microecosystems. Metabolism of the saltwater cells was heterotrophic under all conditions of freshwater input. The upstream communities were adapted to a dependency on allochthonous materials input for production and respiration maintenance. Retarding freshwater input resulted in tying up larger portions of the nutrient pool within the systems in living components. Addition of industrial effluent increased the community maintenance requirements. Retarding freshwater input acted as an environmental stress on the first three cells of the microecosystems. Magnitudes of production and respiration were significantly lower, and zooplankton standing crops and species diversity decreased significantly. Addition of industrial effluent produced similar effects. Decreased freshwater input rate (primary stress) rendered the receiving communities more susceptible to the industrial effluent addition (secondary stress). The organismal composition of the microecosystems was qualitatively similar but quantitatively dissimilar to the organismal composition of Trinity Bay. Because of fundamental similarities in all living systems, the observed responses might be especially representative of those which would occur in Trinity Bay if subjected to similar hydrological alterations.