Effects of suspended solids on the marine environment

The major effects of suspended solids on the marine environment are indirect:

• Suspended solids transport nutrients and heavy metals lightly adhering to the surfaces ('adsorbed') of sediment grains. These are released slowly once the sediment has settled in the estuary, coastal wetland or nearshore waters.

• Reduction of light levels, affecting all life forms in coastal waters dependent on photosynthesis, notably coral zooxanthellae, seagrass and algae. More directly, suspended solids may alter seabed levels and sediment grain sizes, they can smother sedentary life forms and, where stirred up, can clog gills.

Nutrients

The nature and quantities of nutrient pollutants varies greatly from catchment to catchment, depending on land use, geology, and annual water regime; this is illustrated by CSIRO estimates of diffuse source pollution from various land uses on Sydney's western urban fringe (see table 3.8). The high generation rates for recently sewered urban land and disturbed land is striking, and illustrates that an early effect of development is to contribute a sharp rise in nutrients to run-off, as well as a peak of sediment in the flow.

It should be emphasised that a change in nutrient levels in surface waters following land use change varies greatly with both soil type and management practice. Levels of fertiliser application are now worked out carefully because of the long-term rise in fertiliser costs, but the retention of nutrients within the paddock may also depend on subtleties of farm water management, varying

Table 3.8 Diffuse source nutrient generation rates for various land uses in the Hawkesbury-Nepean Basin

Land use

Phosphorus (kg/ha/year)

Nitrogen (kg/ha/year)

Bushland

0.10 [+/-1 0.10

1.50(+/-] 0.50

Established sewered urban

1.30 [+/-] 0.40

5.0 [+/-] 2.0

Recent sewered urban and disturbed

20 [+/-] 10

63 [+/-] 40

Unsewered peri-urban

0.60 [+/-] 0.30

4.0 [+/-] 3.0

Industrial and commercial

1.8 [+/-] 0.40

6.0 [+/.] 2.0

Intensive vegetable growing

8.0 [+/-] 4.0

8.0 [+/.] 3.0

Orchards

0.30 [+/-] 0.20

4.70 [+/.] 3.0

Turf farming

8.0 [+/-] 4.0

8.0 [+/.] 3.0

Fertilised grazing

1.25 [+/-] 0.50

8.0 [+/.] 4.0

Unfertilised grazing

0.25 [+/-] 0.10

0.90 [+/.] 0.50

Extensive agriculture

2.50 [+/.] 2.30

12.50 [+/.] 12.50

Source: Marston 1993

greatly from one farm to another. For example, laser levelling on the Lower Murray floodplain grazing lands allows precise adjustment of flood irrigation to minimise return of irrigation waters (and nutrients) to the river, but many farmers do not carry this out. There is a range of social and economic factors leading to the adoption or rejection of this particular practice

The effects of raised nutrient levels on coastal and estuarine waters were discussed earlier in this chapter. Algal and dinoflagellate blooms, and epiphytic algal growth have been widely reported. Zann (1995, p.ll) identified raised nutrient levels as a particular concern in many nearshore seagrass meadows.

 
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