Wilson Inlet 5 - nutrients

The Wilson Inlet Catchment Committee Inc (WICC)

Wilson Inlet 5

Cover page
Introduction
Nutrients
Water Quality Cycle
Stratification
Water Quality
Comparative data
Summary of Findings

References

Water Quality in Wilson Inlet from 1995 to 2002

Why are nutrients a focus of study?

Figures 4, 5 and 6 (top to bottom):
Analysing nutrient samples in a laboratory and sampling from the boat in Wilson Inlet with a Hydrolab instrument.
Photos B. Boardman and K Parker.

Nutrients are the essential chemical elements required by plants, algae, bacteria and animals for growth. In Wilson Inlet it has been found that the nutrients whose availability limits the growth of plants and algae are primarily nitrogen and phosphorus. Studies have found that the biomass of Ruppia is probably limited more by the availability of phosphorus and that phytoplankton algae biomass is probably more limited by the availability of nitrogen. In the same way that adding nutrients in the form of fertiliser to a paddock or a backyard improves plant growth, adding nitrogen and phosphorus to Wilson Inlet causes greater plant and algal growth. Nutrient cycles in the Inlet have therefore been studied to find out what processes they are governed by and how they can be managed.

A very simplified diagram of the nutrient cycle in Wilson Inlet is presented below in Figure 8. It is important to note that each of the required nutrients can be present in the water column in forms that the plants and algae are able to use and forms that they are not (Figure 7).

Nutrient Form(s) available to plants and algae Form(s) unavailable to plants and algae
phosphorus orthophosphate, also called dissolved inorganic phosphorus (DIP) or filterable reactive phosphorus dissolved organic phosphorus (DOP), and particulate phosphorus (PP)
nitrogen ammonium and nitrate, together called dissolved inorganic nitrogen (DIN) dissolved organic nitrogen (DON), particulate nitrogen (PN), and nitrogen gas (N2)

Figure 7: Forms of nutrients found in the water that are available for plants and algae to use, and forms that are not.

While it appears complicated, the major points to be derived from looking at the nutrient cycle are that: (1) different nutrient cycling pathways produce nutrients in different forms; (2) the sources of nutrients for the Inlet are the catchment and surface sediments; (3) the sinks for nutrients are the ocean and deep sediments, and also the atmosphere in the case of nitrogen only; (4) nutrients contribute not only to Ruppia, epiphyte, phytoplankton and macroalgal productivity, but also to the fishery productivity; and (5) there is the potential for the recycling of nutrients between the surface sediments, water column and the plants and algae, completely bypassing the other sources and sinks.

Figure 8: Simplified nitrogen and phosphorus cycle showing sources and sinks. The sizes of the arrows are intended to give a very rough idea of the relative scales of the different nutrient flow paths. The nutrient sinks are drawn in blue, the catchment source in red and the internal recycling in black.