The influence of nutrient concentration on algal biomass and invertebrate communities in agricultural streams : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Ecology at Massey University, Palmerston North, New Zealand

Abstract

High nutrient inputs have generally been identified as responsible for the degradation of lowland rivers and lakes in New Zealand and internationally. Nutrients have been shown to influence algal community growth rate and composition. In turn algae can have strong effects on invertebrate communities (density, richness, composition, distribution, structure and function). This study investigates the effect of nutrients on algal biomass and higher trophic levels to determine the importance of nutrient loading on stream ecosystems. Twenty six agricultural streams were surveyed in the Manawatu region in February, 2002. Algal biomass was greater in streams with high nitrate levels. Invertebrate communities differed in terms of the quantitative macroinvertebrate community index (QMCI), Ephcmeroptera, Plecoptera and Trichoptera (EPT) individuals and taxa between sites with high and low algal biomass. Regression analysis was used to relate the "quality" of the invertebrate community to stream algal biomass. At 13.μg/cm2 of chlorophyll a there was a dramatic shift in invertebrate community composition to more pollution tolerant taxa. In the Hawke's Ray region nutrient concentration was experimentally increased in 3 low order streams in the summer of 2002/2003. Increased nutrient concentration did not affect stream algal biomass. There were however changes in the proportions of EPT in the enriched community. I propose that these changes in EPT were in response to increased algal growth rates and constrained any increase in algal biomass. Therefore changes in landuse intensity may affect invertebrate community structure