Drivers of Macrophyte Assemblage Structure in Southeast Queensland Streams

Abstract

Recent drought and population growth along the east Australian coastline, combined with water quality issues and degradation of rivers of national significance, have focused attention on the importance of sustainable water resource use and maintenance of the ecological integrity of riverine ecosystems. Central to the development of stream management practices that minimise or ameliorate anthropogenic degradation of riverine ecosystems is an understanding of the principal drivers of ecosystem structure and function. This thesis investigates the principal drivers of submersed macrophyte assemblage structure in streams of southeast Queensland, Australia. Fundamental to this thesis is the development and testing of a conceptual model of macrophyte assemblage structure in streams with disturbance (hydrology and hydraulics) and resource availability (nitrogen, phosphorus, carbon, light) as the principal axes of the model. The suitability of the conceptual model as a framework to describe the key drivers of macrophyte assemblage structure within the study area was explored by testing hypotheses of macrophyte assemblage structure at various spatial scales across disturbance and resource availability gradients. In particular, it was hypothesised that assemblage structure could be described in terms of appropriate measures of disturbance and resource availability; that extremes of disturbance (e.g. flooding) and resource availability (e.g. high shade) would limit macrophyte growth; and that species richness would vary according to the intermediate disturbance hypothesis. Furthermore, it was expected that flow regime changes resulting from flow regulation would have predictable impacts on macrophyte assemblage structure, in line with these hypotheses. In general the results of this thesis supported hypotheses based on the conceptual model. Spatial and temporal patterns in macrophyte assemblage structure were associated with hydrologic-hydraulic and resource availability gradients. These patterns were shown to be consistent across a variety of spatial scales, but were tighter at the transect and hydraulic unit (riffle, run, pool) scales. The key descriptors of the disturbance axis of the conceptual model were bankfull shear stress, substrate stability, the coefficient of variation of mean daily discharge, days since last flood and the number of floods in the 12 months prior to sampling. Riparian canopy cover and alkalinity were identified as the key descriptors of the resource availability axis. However, geomorphological attributes such as bankfull depth and channel orientation were also important in structuring macrophyte assemblages. From the conceptual model it was predicted that flow regulation by large dams would result in increased macrophyte abundance due to reduced disturbance frequency and magnitude. Further, it was predicted that reduced temporal hydrologic variability in sites with regulated flows would produce relatively stable macrophyte assemblages as disturbance frequency (i.e. the frequency of biomass loss) should be minimal compared with an unregulated stream of similar morphology and water quality. These predictions were tested in two ways. Firstly, spatial patterns in assemblage structure were compared between regulated and unregulated sites. Flow regulation had no effect on species richness but macrophyte cover was higher than expected in regulated sites compared to macrophyte cover in unregulated reference sites. The extent of increase in macrophyte cover was dependent upon site location in the catchment, as hypothesised in the Serial Discontinuity Concept, and local (site-specific) characteristics. In the lower Brisbane River, the increase in macrophyte cover was relatively small as large macrophyte beds were natural features of this lowland river with limited riparian shading. In Yabba Creek, a mid-catchment tributary, the increase in macrophyte cover was relatively high, taking the form of extensive macrophyte beds which were uncommon in comparable (unregulated) streams characterised by high riparian canopy cover. Secondly, the effect of flow regulation on temporal variability in macrophyte assemblage structure was examined over a 12 month period in a regulated stream (Yabba Creek) and an adjacent unregulated stream (Amamoor Creek). The magnitude of temporal environmental variability was similar for both creeks. Macrophyte assemblage structure was found to be highly variable in Amamoor Creek, and comparatively stable in Yabba Creek, in accordance with conceptual model predictions. Again it was found that local habitat conditions in each waterway (particularly riparian shading) had an overriding influence on the response of submersed macrophytes to hydrologic variability and flow regulation. The effects of riparian shading on macrophyte assemblages were investigated further by examining the influence of riparian canopy cover and light availability on the growth of five aquatic macrophyte species in a stream rehabilitation site. This site was located in a second order headwater stream with a degraded riparian zone where unshaded parts of the site were infested with submersed and emergent macrophytes. Changes in assemblage structure were monitored over a two year period and related to changes in riparian canopy cover resulting from the regrowth of native riparian tubestock. Reductions in macrophyte abundance and changes in species composition were predicted from the conceptual model as riparian shade increased due to vegetation regrowth. The results are discussed in light of hypotheses derived from the conceptual model, particularly with respect to the effects of interactions between flow regulation and resource limitations on macrophyte assemblage structure. The results of this thesis show that the conceptual model is a valid representation of the physical habitat template occupied by submersed macrophytes in the study area, and is also likely to be valid for lotic ecosystems in other bioregions, provided that herbivory is not a major (alternative) source of disturbance. However, the contribution of hydrology as the principal direct driver of macrophyte assemblage structure is questioned. This thesis has shown that local (site-specific) conditions, including riparian canopy cover and substrate stability, are probably more important drivers of macrophyte assemblage structure per se than hydrology itself. Local habitat conditions such as bed stability and substrate composition determine how features of the flow regime, such as discharge magnitude, influence macrophyte assemblage structure. Management strategies that focus solely on manipulating the hydrologic regime (e.g. by provision of environmental flows) may not fully accommodate the habitat template for submersed macrophytes in lotic ecosystems.