|Title||Nutrient heterogeneity and the maintenance of species diversity: periphyton response to substratum and water enrichment in a nutrient-poor stream|
|Year of Publication||1985|
|Academic Department||School of Natural Resources|
|Degree||Doctor of Philosophy|
|Number of Pages||150 pp.|
|University||University of Michigan|
|City||Ann Arbor, MI|
The effect of nutrient heterogeneity on the distribution of planktonic algae in lakes and oceans has received considerable attention. Individual algal cells experience different rates of resource supply due to depth gradients of nutrients and light, turbulent upwellings, and other mixing processes. It has also been shown that small scale nutrient patchiness caused by fish and zooplankton excretion are important to phytoplankton. Additionally, recent emphasis has been placed upon nutritional relationships between attached algal communities in lakes and the substrata on which they grow. In contrast, information concerning effects of nutrient patchiness on the distribution of periphyton in streams is virtually nonexistent. Because of the influence of current, streams have historically been considered to undergo little local nutrient recycling, with effects of upstream and downstream recycling masked by nutrient renewal from watershed runoff or groundwater. The physiological richness of running water has perhaps been classically misinterpreted as meaning "nutritionally rich." As pointed out by Elwood et al., though total quantities of nutrients transported in streams draining undisturbed watersheds may be high, concentrations of dissolved nutrients are often low. Studies in the last two decades have shown that nutrient supply plays a major role in limiting algal productivity in a wide variety of lotic systems. Furthermore, at low ambient nutrient concentrations periphyton communities are often diffusion limited. If periphyton populations are in contact with a highly concentrated nutrient source, however, diffusion limitation from overlying waters may not exert a primary effect. Previous studies in a stream in northern Michigan related high diatom biomass and taxon distribution patterns on sand grain, chironomid retreats (Diptera: Chironomidae) to localized nutrient release. Chironomid retreats cover most of the submerged wood in the study stream, influencing biomass and patchiness of periphyton communities. Interest in causal factors behind those distribution patterns has led to the present study. The first chapter examines how nutrient content of a substratum affects stream periphyton standing crop and community structure. A technique was designed that allowed distinction between effects of substratum texture and nutrient content. Artificial sand-agar substrata simulate the structure and texture of chironomid tubes and can be differentially enriched. The second chapter investigates how nutrients in stream waters interact with nutrients in benthic substrata to determine periphyton growth, succession and physiognomy. This section also examines the importance of substratum-water interactions: (1) in determining natural periphyton distribution patterns, and (2) in evaluating nutrient limitation and cycling in streams. To address these questions, periphyton response to substratum fertilization versus enrichment of overlying water was experimentally separated by combining the substratum enrichment technique (Chapter 1) with an in situ flow-through bioassay system. The third chapter examines: (1) whether the same periphyton taxa growing on two different substratum types will respond in a similar fashion to nutrient enrichment, and (2) if substratum specificity will break down in response to enrichment.