Invertebrate larval dynamics in seasonally closed estuaries

Estuarine benthic assemblages are often numerically dominated by polychaetes. The limits of these populations are determined by larval, and probably to a lesser extent adult movement. A previous study (Newton 1996), indicated that planktonic polychaete larvae were very abundant over the summer months in the Hopkins River; however, the identification and source of these larvae was not known. Defining the extent of a population, and therefore the likelihood of that population recovering following a perturbation, is crucial for effective estuarine management. This study investigated both the likely source of the larvae, (i.e. estuarine or marine) and the extent of larval dispersal within and between estuaries by addressing the following questions: Which taxa produced the planktonic larvae? Are these taxa resident estuarine species? Are the larvae of different taxa evenly distributed within the estuary or do physicochemical parameters or other factors influence their abundance? Are the same larvae found in other estuaries along the coast? and Is there exchange of these larval taxa with the marine environment and other estuaries? Larvae were identified and described by culturing commonly occurring planktonic larvae until adult characteristics appeared. The spionids, Carazziella victoriensis and Prionospio Tatura, numerically dominated the plankton in the Hopkins and the spionid, Orthoprionospio cirriformia was recorded from the Hopkins, Curdies and Gellibrand estuaries. Two spionids, Carazziella sp. and Polydora sp. were identified from tidal waters. Mouth status and physicochemical conditions (salinity, temperature and dissolved oxygen) were monitored in each estuary. Whereas the Merri and Gellibrand estuaries were predominantly stratified over the sampling period, the Curdies was more often well mixed and the Hopkins varied from well mixed to stratified. The duration of mouth opening and hence the opportunity for larval exchange also varied in each estuary. The Merri River was closed for 13.5% of days over the study period, the Gellibrand River for 18.4%, the Hopkins River for 49% and the Curdies River for 71.0%. The distributions of larvae at spatial scales of metres, 100s of metres and kilometres were investigated within a single estuary. While the same larvae, C. victoriensis, P. Tatura and bivalve larvae, were found along the length of the Hopkins estuary the abundances varied at different spatial scales suggesting different processes were influencing the distribution of P. Tatura larvae, and C. victoriensis and bivalve larvae. The distribution of larvae between several estuaries was investigated by monitoring meroplankton at two sites at the mouth of each of the four estuaries approximately monthly (except for winter months). Different meroplanktonic assemblages were found to distinguish each estuary. Further, C. victoriensis and P. Tatura larvae were only recorded in the Hopkins but larvae of the spionid, Orthoprionopio cirriformia were detected in the Hopkins, Curdies and Gellibrand estuaries. The extent of larval exchange with other estuaries and the marine environment was determined by monitoring tidal waters. Settlement trays were also deployed to determine if larvae were moving into estuaries and settling but not recruiting. P. tatura larvae were not detected in the tidal waters of any estuary and while C. victoriensis and O. cirriformia were found in both flood and ebb tides there was no evidence of movement of theses taxa to other estuaries. Larvae of the spionids, Carazziella sp. and Polydora sp., were found in tidal waters of each estuary but were rarely detected in the plankton within the estuaries. Neither species was found as an adult in background cores from any estuary, nor with the exception of a few individuals in the Merri, were they detected in settlement trays in any estuary. I conclude that the source of the larvae of C. victoriensis, P. Tatura and O. cirriformia is estuarine and while C. victoriensis, and O. cirriformia move in and outh of the source estuary in tidal waters there was no evidence for movement to other estuaries. The spionids, Carazziella sp. and Polydora sp were considered to be marine and while they moved in and out of estuaries in tidal waters they did not usually settle in the estuaries. The results of this study are a crucial first step in the development of ecological models to better understand dispersal in seasonally closed estuaries that are typical of southern Australia. This study emphasises the unique physicochemical characteristics and biological assemblages within these estuaries and the need for estuarine management to reflect these differences.

Impact of stresses imposed on macroinvertebrate communities in two suburban streams

The aim of the project was to determine factors which explain the distribution of macroinvertebrates in two Melbourne streams both drastically affected by urbanisation. A detailed description is given of Dandenong Creek, flowing through the south-eastern suburbs, and Darebin Creek, in the northern suburbs, emphasising stream features likely, or known, to influence the drift and benthic fauna. Faunal sampling was carried out in Dandenong Creek from June 1992 until July 1993, and in Darebin Creek from February 1995 until March 1998. Physicochemical parameters were also recorded. The collected data, together with previously existing data, were analysed using multivariate analyses: non-metric multi-dimensional scaling (NMDS); analysis of similarities (ANOSIM); matching biotic and abiotic variables using BIOENV, and principal component analysis (PCA). Various biotic and diversity indices were calculated in an attempt to identify the major factors responsible for the failure of the fauna to recover from previously more seriously degraded water quality. The contribution of drift to the colonisation potential in Dandenong Creek appeared to be impacted by retarding basins, underground barrel-draining and channelization. Results also indicated that increased conductivity adversely affected the fauna in the lower reaches of Dandenong Creek. It was concluded that in Darebin Creek, high nutrient levels, as well as other pollutants, had resulted in low macroinvertebrate diversity in both the drift and benthos. If, as this study suggests, faunal diversity is a valid measure of stream health, the following factors need to be addressed for catchment-wide, stream management: lack of riparian zone vegetation (increasing bank erosion and making the benthic habitat unstable, with greater temperature variability); control of stormwater runoff (flow variability, increased conductivity, nutrient levels, sediment loads, sewage effluent, industrial discharges and heavy metals), and to modify retarding basins to increase stream continuity.