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The Water Quality of the Blackmore Estuary., Darwin Harbour: May and June 2001



The Water Quality of the Blackmore Estuary., Darwin Harbour: May and June 2001


Dixon, I. H.; Padovan, Armando V.


E-Publications; E-Books; PublicationNT; Report No. 19/2003




Blackmore Estuary


The primary aim of this study was to obtain data on the water quality of the upper reaches of Blackmore Estuary in Darwin Harbour at a time when development in the catchment is relatively minor. Future development within the Blackmore Estuary catchment may alter the quality of water flowing into the estuary and in turn the main body of Darwin Harbour. Results from this study can be used as a baseline data set, against which to measure any future changes.; Made available via the Publications (Legal Deposit) Act 2004 (NT).

Table of contents

Summary -- Introduction -- Methods -- Results and discussion -- Conclusions -- References -- Appendix A-B




Salinity; Water clarity; Nutrients; Algae

Publisher name

Northern Territory. Department of Infrastructure, Planning and Environment

Place of publication



Report No. 19/2003

Now known as

Northern Territory. Department of Environment and Natural Resources


iii, 69 pages : some colour illustrations, some colour maps, tables ; 30 cm.

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3 SUMMARY In May and June 2001, waters of the Blackmore Estuary were sampled for physical properties, water clarity, nutrients and algae. Eleven sampling locations were established along the main channel of the estuary and five of its major tidal tributaries. At each location, 2-4 sites were sampled across the width of the channel, depending on channel size. On the second sampling occasion, physical properties were measured down the water column. Results were analysed for temporal and spatial patterns. These patterns included gradient changes over the length of the estuary, across the channel, down the water column and over time. The majority of properties measured varied across the width of the channel. Nitrate concentrations and turbidity did not vary, and suspended solids and euphotic depth showed minor variations across the channel width. Salinity levels had a strongly defined pattern, where salinity increased with distance from freshwater inflows. Berry Creek was identified as having the lowest salinity (7 ppt) in the estuary and also the most stratified water column. As freshwater inflow to the estuary decreased between May and June, salinity levels over the estuary increased. At the most seaward sampling location, salinity (30 ppt) was lower than those found in a separate study of the main body of Darwin Harbour (35 ppt). This indicated that at all sampling locations, freshwater inflow was continuing to influence estuary waters on both sampling occasions. Temperature, pH and dissolved oxygen shared similar spatial and temporal trends. They showed little or no difference over time and were generally lower in the upper reaches than in the main channel of the estuary, except in Berry Creek, which had the highest recorded temperature (29 C) in the estuary. Down the water column, temperature and dissolved oxygen decreased with depth, however pH remained constant. Water clarity was assessed from measurements of turbidity, suspended solids and euphotic depth. The estuarys water was of relatively high clarity (turbidity <12 NTU) and generally decreased between May and June. Water clarity was generally lower in the tidal tributaries than the main channel of the estuary, which in turn had lower clarity than the main body of Darwin Harbour. Factors that determine these trends are considered to be water depth, sediment size and proximity to mudflats. The estuary waters were low in concentrations of nitrogen and phosphorus. Organic nitrogen made up the bulk (62-96%) of the total nitrogen pool, followed by nitrate (typically <15%). Total nitrogen generally increased in concentration between May and June. Concentrations of nitrogen and phosphorus were generally higher in the tidal tributaries and decreased with distance from freshwater inflows. Compared to nutrients measured in the main body of Darwin Harbour, Blackmore estuary generally had higher concentrations of nitrogen and phosphorus. Chlorophyll a was measured as an indicator of algal biomass. Concentrations in the main channel were similar to those measured in the main body of Darwin Harbour (typically <2.5 g L-1). The upper reaches of the estuary had higher concentrations (up to 4.4 g L-1) and showed the greatest decrease between May and June.