The Effects of Biological and Physical Disturbance on Rainforest Stream Algal Communities

Abstract

In this study, the effects of biological and physical disturbance on algal assemblages on cobbles in rainforest streams were investigated. Study sites in two rainforest streams, Stony Creek and Booloumba Creek, located in south- east Queensland, were used for both a regular sampling program and an extensive experimental schedule. Previous research has shown that physical disturbance due to spates and, in the absence of high discharge events, biological disturbance due to invertebrate grazing exert significant structuring and controlling forces on periphyton assemblages in streams. The principal objectives in this study were to deternine the relative importance of physical disturbance in the form of spates and desiccation, to outline the significance of biological effects, in particular as a result of grazing pressure, and to describe algal succession on substrates cleared after disturbance events. Periphyton chlorophyll a, biomass values and grazer numbers were monitored over a period of two years in relation to rainfall and stream discharge (to pinpoint the incidence of spates). Periphyton assemblages were composed mainly of diatoms (Cocconeis and Navicula being the dominant genera). Physical disturbance by spates was clearly an important factor structuring stream algal assemblages. Periphyton chlorophyll a and biomass values were reduced following a high discharge event, but then recovered relatively fast in the absence of any other disturbance. Furthermore, chlorophyll a values were correlated with grazer numbers. Differences were observed in the epilithon of Stony Creek and Booloumba Creek: while cobbles in the former supported more chlorophyll a containing material of algal origin and less non-algal biomass, the reverse was true for latter stream. This was most probably due to the physical characteristics of the two stream channels, primarily their slope, which was considerably steeper in Stony Creek, thus requiring less depth to facilitate a major disturbance event. This means that the physical characteristics of streams are most important determining factors regarding the effects of high discharge events, with the disturbance regime playing a significant role in the composition of the stream benthos. However, some major changes in chlorophyll a and biomass occurred in the absence of big disturbances. Periphyton on cobbles collected in the streams was subjected to various time periods out of the water to determine the effects of desiccation. Diatom assemblages on cobbles were not resistant to desiccation and did not recover following re-immersion in the streams. Thus, periods of low flow are capable of reducing periphyton assemblages, however, this type of disturbance would take place over a longer period compared to spates. A colonization experiment where polyethylene foils were introduced into the streams was used to monitor algal succession on bare substrates. Diatoms were the only algae which colonized experimental foils, exhibiting distinct successional patterns in both streams. A series of grazer manipulation experiments were employed to test for any effects of grazing; this involved the inclusion / exclusion of caddisfly larvae from cobbles enclosed in screened perspex cylinders. Signficantly, the effects of grazing invertebrates resulted in an increase in chlorophyll a and biomass values, contrary to expectations and the majority of past studies. This could mean that grazers at a certain density may actually stimulate algal growth and there exists a fine balance between when grazer numbers enhance periphyton productivity or periphyton is overgrazed. Overall, results obtained indicated that variations in stream algal assemblages were a consequence of the combined effects of spates, low flows and, to a lesser extent, grazing.