Determining the ecological health of estuaries in North-Western Tasmania: Montagu River

Brief description

This study considered a range of water-column and sediment (benthos) based variables commonly used to monitor estuaries,utilising estuaries on the North-West Coast of Tasmania (Duck, Montagu, Detention, and Black River). These included: salinity, dissolved oxygen, turbidity, nutrient and chlorophyll a levels for the water-column; and sediment redox, organic carbon content, chlorophyll a and macroinvertebrate community structure amongst the benthos. In addition to comparing reference with impacted estuaries, comparisons were also made across seasons, commensurate with seasonal changes in freshwater river input, and between regions within estuaries (upper and lower reaches) - previously identified in Hirst et al. (2005). This design enabled us to examine whether the detection of impacts (i.e. differences between reference and impacted systems) was contingent on the time and location of sampling or independent of these factors. The data represented by this record was collected in the Montagu River.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Estuaries were also divided into clearly definable upper and lower regions, with 3 sites within each region sampled. Each estuary was visited on four occasions during the course of this study to measure a range of water quality (salinity, dissolved oxygen, turbidty, nutrients, chlorophyll a), benthic (sediment organic carbon, redox and chlorophyll a) and biological (macroinvertebrates) parameters. Estuaries were initially visited in October 2005 (spring) followed by visits in January (summer), April (autumn) and July (winter). Nutrient dynamics and physicochemical status of the water column With the exception of salinity and dissolved oxygen all water column parameters were measured at low tide. This enabled standardization of measurements in relation to tidal phase, whilst measuring nutrient levels at their maximum concentrations (see Hirst et al. 2005). Variables measured included (NB: all measurements were made in surface waters at LT unless otherwise stated): 1. Salinity (surface and bottom measurements) at low and high tide 2. Dissolved oxygen concentration (surface and bottom measurements) at low and high tide 3. Turbidity 4. Temperature 5. Dissolved nutrient concentrations including ammonium (NH4-N), nitrate and nitrite (NOx-N) and soluble reactive phosphorous (P) 6. Chlorophyll a concentrations Measurements were made mid-channel (i.e. equidistant from either bank where possible) within surface waters <30 cm depth. Salinity, DO and turbidity were measured in the field using meters. Nutrient levels were determined later in the lab from water samples collected in the field. Water samples were stored on ice in the field then later frozen. Soluble ammonium, nitrate, nitrate and reactive phosphorous were analysed by Analytical Services Tasmania. Chlorophyll a levels were determined by filtering water samples in the field onto GF/F Whatman filters, chlorophyll extracted from filters with 90% v/v acetone (90% v/v) over a 24 hour period in darkness at 4 °C, then determined spectrophotometrically. Chlorophyll a levels were used as a proxy for water-borne microalgal biomass in this study. Sediments Sediment organic carbon, redox potential and chlorophyll a levels were determined for sediments adjacent to the water column sampling sites. Samples were taken just below the low water mark at low tide. Sediment organic carbon content was derived from 45 mm diameter cores to a depth of 50 mm; sediment chlorophyll a from three replicate micro-cores 22 mm diameter inserted to a depth of 15 mm (following Light and Beardall 1999). Sediments were frozen in the field and returned to the laboratory for analysis. Organic carbon content of sediments was determined via chemical titration. This has been found to be a more accurate method than the loss on ignition (LOI) method used previously in Hirst et al. (2005). Chlorophyll a was measured as a proxy for microalgal biomass in the sediments. Chlorophyll was extracted from the sediments with acetone (90% v/v) over a 24 hour period in darkness at 4 °C, and then determined spectrophotometrically. Sediment from which chlorophyll a was extracted was dried at 80 °C for 48 h and weighed allowing for chlorophyll a concentration to be expressed as μg of chlorophyll a per g of dried sediment (ug g-1). The redox potential of the sediment was measured in situ using a pH/redox probe at a depth of 30 mm below the surface. Redox potential was corrected for differences in ambient temperature between sampling dates prior to analysis. Benthic macroinvertebrates The diversity and composition of the soft sediment macroinvertebrate fauna in each estuary was determined using replicated sediment cores collected at each site. At each site five sediment cores (diameter = 150 mm, depth = 100 mm) were collected from subtidal sediments using a transect spanning from the low water mark to depth of approximately 0.7 m as a guide. Cores were collected at 0.0, 0.1, 0.2, 0.5 and 0.7 m depths. Cores were then sieved through a 1.0 mm-mesh sieve in the field and the portion retained was fixed in 5% buffered formalin. Salinity and DO measurements corresponding with the scale of the macroinvertebrate sampling were also made by resting the probes on the sediment. This information was later used to examine correlations between macroinvertebrate assemblage structure and environmental variables. Samples were sorted in the lab to the lowest possible taxonomic level and the number of individuals of each taxa recorded. This information was used to calculated species richness (i.e. no. of taxa), total faunal abundance and species composition for each site by amalgamating the replicate samples (i.e. totals not means).

Notes

Credit
Natural Heritage Trust (NHT) Project No. 52508

Credit
Natural Resource Managment (NRM) Cradle Coast

Purpose
Here we utilize a comparative approach where putative impacts within two estuaries (Duck and Montagu River) are determined via comparison with two reference estuaries deemed to be relatively free of human impacts (Detention and Black River).