Changes in agricultural intensity and river health along a river continuum

Abstract

1. The impact of agricultural activities on waterways is a global issue, but the magnitude of the problem is often not clearly recognized by landowners, and land and water management agencies. 2. The Pomahaka River in southern New Zealand represents a typical lowland catchment with a long history of agricultural development. Fifteen sites were sampled along a 119‐km stretch of the river. Headwater sites were surrounded by low‐intensity sheep farming, with high‐intensity pasture and dairying occurring in the mid‐reach and lower reaches. 3. Water clarity decreased significantly from about 6 m in the headwaters to less than 2 m in the lower reaches. Benthic sediment levels increased significantly downriver, peaking at 35 mg m−−2 below several tributaries with high‐intensity agriculture in their catchments. Periphyton levels were also significantly greater in the lower reaches than the headwaters, and coincided with increased nitrogen (DIN) and phosphorus (SRP) concentrations. 4. Macro‐invertebrate species richness did not change significantly throughout the river, but species composition did with Ephemeroptera, and to a lesser extent, Plecoptera and Trichoptera dominating the headwater sites (where there was high water clarity, and low nutrient and periphyton levels). Downriver these assemblages were replaced by molluscs, oligochaetes and chironomids. 5. Canonical correspondence analysis indicated that agricultural intensity and physical conditions associated with agriculture activity (e.g. impacted waters, high turbidity and temperature) were strongly associated with the composition of benthic assemblages at differing reaches down the Pomahaka River. 6. The present results indicate that quantifying agricultural intensity within a catchment, particularly relative livestock densities, may provide a useful tool for identifying threshold levels above which river health declines.

Longitudinal changes in biota along four New Zealand streams: Declines and improvements in stream health related to land use

“…These streams had increasing nutrient concentrations and fine sediment cover downstream; biotic indices declined downstream and were closely related to fine sediment cover. similar declines in stream health have been seen in other catchments with increasing agricultural intensity in the downstream direction (DeLong et al 1997;harding et al 1999). some changes in macroinvertebrate communities may also be rwith NMDS 1 expected downstream because of natural changes in physical characteristics, such as decreasing substrate size, and sources of organic matter (vannote et al 1980).…”

Species traits reveal effects of land use, season and habitat on the potential subsidy of stream invertebrates to terrestrial food webs

“…This likely reflects higher autotrophic productivity associated with reduced shading and increased nutrient inputs in agricultural streams (Delong and Brusven 1998;Harding et al 1999), particularly given there were no differences in the overall size of the two stream types (Carlson et al 2016). Abundances of most of our focal species traits were similarly higher in the .9 n.s 0.1 n.s 1.8 n.s 1.0 n.s 0.8 n.s 0.0 n.s 0.9 n.s 10.6 0.014 LxS 9.9 0.016 3.3 n.s 1.9 n.s 6.6 0.037 8.0 0.026 0.0 n.s 7.5 0.029 0.1 n.s LxSxH 3.7 n.s 2.7 n.s 3.8 n.s 3.1 n.s 3.3 n.s 0.0 n.s 3.7 n.s 0.4 n.s agricultural streams, and in riffle habitats.…”

Assimilatory uptake rather than nitrification and denitrification determines nitrogen removal patterns in streams of varying land use

“…In combination with the high light levels in these streams, channel geometry and hydrology encourage the growth of tall grasses such as Phalaris arundinacea on channel edges, which may ultimately restrict light availability during summer. Also, high turbidity often associated with agricultural streams may further reduce light reaching the stream bottom (Harding et al 1999).…”