PERIPHYTON DEVELOPMENT IN 3 VALLEY SEGMENTS OF A NEW-ZEALAND GRASSLAND RIVER - TEST OF A HABITAT MATRIX CONCEPTUAL-MODEL WITHIN A CATCHMENT

We monitored water nutrient concentrations, periphyton biomass, and pe riphyton cellular nutrient concentrations in run and riffle habitats a t sites representative of headwater, mid-catchment and lowland valley segments of a grassland river for two years, and community composition on four occasions during this period, to determine how these communit ies varied spatially and temporally as a function of downstream change s in hydraulic conditions, flood disturbance regimes and enrichment. P redictions of community biomass and structure were tested under a prop osed habitat matrix conceptual model for periphyton. Discharge increas ed and valley segment slope decreased in a downstream direction as exp ected from geomorphic models of catchment processes. However, site-spe cific depths and velocities did not change systematically down the riv er. Water nutrient concentrations suggested a change from nitrogen lim itation of primary production at the headwater site to phosphorus limi tation at the lowland site. This was associated with a downstream incr ease in stream nitrate concentrations, but not phosphorus. However, ce llular nutrient concentrations and instream nutrient bioassays suggest ed either N limitation, or weak N and P limitation, at all sites. In r uns, mean monthly cklorophyll-a did not vary significantly (P>0.05) am ong the three sites, but it did in riffles. The differences in communi ty biomass between run and riffle habitats were more significant than differences among sites. Periphyton community structure in the runs wa s similar among valley segments during a period of frequent flood dist urbances, but during a period of infrequent disturbances communities i n the headwater sites were dominated by filamentous cyanobacteria wher eas in the mid-catchment and lowland sites they were dominated by nitr ogen-fixing cyanobacteria and diatoms. Major differences in community structure occurred in riffles among the sites. Riffle communities at t he headwater site were dominated by filamentous cyanobacteria and diat oms, whereas at the mid-catchment and lowland sites filamentous green algae and diatoms were dominant, possibly a result of increased nitrat e concentrations progressing downstream. Overall, a downstream gradien t in nitrate enrichment appeared to control biomass and community comp osition of the riffle communities, but not the run communities. Strong grazer activity in runs during more hydrologically stable periods at the lowland site appeared to override any response to nutrients and ma intained biomass at low levels. Periphyton biomass and community compo sition was predicted more accurately from the habitat matrix conceptua l model based on local habitat factors than by expected downstream gra dients in hydraulic conditions and enrichment.