Sedimentología

107 Figure 5: Land use in the Manawatū River catchment (in hectares)

(Source: Manawatū River Leaders Forum, 2011) Effects of nutrients

Nutrients and sediment make their way from farms into water in several ways. Sheep, beef cattle, and dairy cows deposit nitrogen on the land through their urine. The pasture and soil cannot assimilate all this nitrogen, so the excess either leaches through the soil into groundwater, or runs off it in wet conditions into surface water (Parliamentary Commissioner for the Environment, 2012). Phosphorus enters the water attached to soil, for example, when it slips off the land as a result of erosion (R. Death, 2012a). Once these nutrients are in the water, they fertilise the plants and make them grow. There are three types of

60% 18%

21%

1%

108 aquatic plant, one of which is periphyton (Parliamentary Commissioner for the Environment, 2012). In small quantities, periphyton47 _{is the base of the river}

food web and an essential food source for other forms of aquatic life; it is ‘the grass of streams for aquatic grazing animals’ (Biggs, 2000, p. 25). Periphyton is a community of organisms composed of algae, bacteria, diatoms, fungi, and other microbes. The composition of that community changes with varying environmental conditions. Macro-invertebrates graze on the periphyton and fish and animals on the riparian margin, such as spiders, bats and birds, eat the macro-invertebrates48 _{(R. Death, 2012b).}

Excessive periphyton growth, however, is problematic for several reasons. It can be dangerous for wading, it can proliferate into toxic blooms, and it can be an unpleasant nuisance when swimming or carrying out other recreational activities (see Table 3 in Biggs, 2000, p. 28). Excessive periphyton growth can also change the river food web by affecting the macro-invertebrates that graze on it. Some macro-invertebrates do not eat periphyton when it becomes overgrown, possibly because it is too big to fit in their mouths (Biggs, 2000). Smaller prey items are then left for the fish to eat (R. Death, 2012b). Consequently, the fish have to use more energy to consume an adequate amount of prey, making them grow slower, skinnier, and more susceptible to disease and death (R. Death, 2012b). Their reproductive success is also reduced.

47 _{The Parliamentary Commissioner for the Environment (2012) defines periphyton as ‘microscopic}

algae, cyanobacteria and bacteria living in freshwater but attached to objects such as submerged rocks, wood or macrophytes [large water plants and algae that are visible to the naked eye]’ (pp. 79– 80).

48 _{Macro-invertebrates are animals that do not have a backbone and are large enough to be seen}

with the naked eye. They include insects, snails and worms (Parliamentary Commissioner for the Environment, 2012).

109 Effects of sediment

Alongside nutrient enrichment, Death (2012a) identifies sedimentation as the primary cause of declining water quality in the region. He asserts that agriculture, particularly on highly erodible land, can cause a 2000% or more increase in the amount of sediment that accumulates in waterways. In the catchment, there are almost 52,000 hectares of unprotected erosion-prone land (Manawatū River Leaders Forum, 2011). Death (2012a) describes several negative effects of sediment on macro-invertebrates and fish. These impacts include: smothering river and stream beds and therefore, the periphyton on which macro-invertebrates graze; filling in the spaces between the stones on the bed that fish and macro-invertebrates live in during the day and shelter in from strong flows, such as those that occur during floods; and making the water turbid. In turbid conditions, fish that are visual feeders, such as trout, find it harder to see and consequently, have to spend more time and energy hunting for prey. The additional time and energy spent finding food can have a detrimental effect on their growth rates, general health, and ability to reproduce.

Trout and many native fish species are especially sensitive to sedimentation. Trout lay their eggs in the beds of rivers and streams. Kier and Associates (2011) explain that the eggs and the newly spawned fish need clean, cold water to deliver oxygen and carry away waste. They also describe the effects of sediment on the eggs and young fish. Fine sediment can cover the stream bed and suffocate the eggs, the young fish, and the macro-invertebrates on which the young fish feed. Larger sediment particles can roll along the stream bed and crush the eggs and the fish. Sediment can also act as an abrasive on the gills and remove the protective mucous on the skin, making the fish more susceptible to

110 infections. For these reasons, protecting trout habitat in the catchment from sedimentation and stream bed disturbance is critical. Death (2012a) recommends a minimum setback distance of 20 metres on the edges of trout spawning rivers to safeguard them from activities that might cause sedimentation. The catchment contains important trout fisheries and spawning areas throughout (Manawatū River Leaders Forum, 2011).

Mitigation methods

Riparian buffer zones, such as those created by minimum setback distances, are an important tool for reducing agricultural pollution of waterways. Such zones provide numerous benefits for freshwater ecosystems (see Parkyn, 2004). Among other things, grassy and forested riparian margins stabilise stream banks and prevent erosion, deliver shade that keeps the water cool and stops light from reaching the streambed where it can promote plant growth, and provide habitat for macro-invertebrates and fish. The grassy banks of the Manawatū River at Whirokino near the river mouth, for example, may be the largest known spawning site for īnanga (the chief whitebait species) in the North Island (see Grocott, 2013).

Land use on, and livestock access to, riparian margins has detrimental impacts on water bodies. For example, fertiliser application too close to the edges of waterways can lead to contaminants entering the water (Death, 2012b). Livestock can damage the banks with their hooves and cause the banks to erode (Parkyn, 2004). One study has shown that dairy cows are 50 times more likely to defecate when crossing a stream than they are when standing on land, causing intense faecal contamination of the water (Davies-Colley, Nagels, Smith, Young, & Phillips, 2004). Livestock that are allowed to graze riparian margins

111 can degrade or destroy the vegetation that provides stream shading and habitat for the terrestrial and aquatic life forms that are an essential part of the river ecosystem (Death, 2012b).

While riparian buffer zones are an important mitigation method, they are limited in that they do not address the much more pervasive problem of nitrogen leaching through the soil after it is deposited there as animal urine. This problem is the result of the number and types of animals on the land. While nitrogen leaching can be mitigated to an extent by farmers using particular techniques, the gains in water quality that might be made from applying these techniques are far outweighed by the scale of nutrients that are lost from vast and increasing areas of pastoral land (Parliamentary Commissioner for the Environment, 2013). In the catchment, Horizons Regional Council argued that controlling certain types of farming was the primary answer to reducing the quantity of nitrogen leaching from farms.