PERIPHYTON RESPONSES TO A HYDRAULIC-GRADIENT IN A REGULATED RIVER IN NEW-ZEALAND

1. Periphyton species composition, chlorophyll a concentration, organi c matter biomass, and metabolic activity were analysed at a site in a regulated river with low nutrient concentrations to investigate popula tion and community level responses to a spatial gradient in hydraulic conditions. The communities were dominated by diatoms over the full hy draulic range [0.1-0.5m depth, 0.1-1.5ms-1 velocity, and 0.01-1.5 Frou de number (Fr)] with Cymbella kappii, Synedra ulna, and Gomphoneis her culeana having the highest relative biovolumes. 2. Unexpectedly, Cymbe lla kappii and Synedra ulna were abundant or co-dominant at all levels of velocity and Fr. Gomphoneis herculeana was most abundant within th e mid-velocity range (velocity = 0.8-1.2ms-1). 3. The physiognomy of t he communities changed with a change in hydraulic conditions. There wa s progressively more diatom mucilage as velocity and Fr increased whic h resulted in a macroscopic change from relatively thin films (1-2mm) at low velocities and Fr (in pool habitats), to thick (approximately 1 0 mm) mats at higher velocities and Fr (in riffle habitats). Associate d with this, ash-free dry mass (AFDM) increased strongly, but chloroph yll a concentration did not, resulting in a decrease in % chlorophyll a over the gradient. 4. The results of experiments conducted in an in situ benthic chamber showed no significant differences in gross primar y production between two communities of different biomass, but indicat ed a slightly decreasing trend with increasingly velocity (0.14 to 0.3 8 ms-1). In contrast, community respiration increased greatly with mat biomass and also as a function of increased water velocity. The combi ned AFDM, chlorophyll a and metabolic results indicated that the zone of photosynthesis was maintained at the surface of the mats, with vari able amounts of mucilage being secreted below, depending on hydraulic conditions. 5. The diatom community had considerable physiognomic plas ticity through the accumulation of mucilage. It is suggested that this high mucilage secretion may be an active feedback mechanism to help a meliorate environmental stress. If so, present theory on velocity cont rol of periphytic algal development in streams, which is essentially b ased on passive responses, needs expanding. It is concluded that the r esponse of periphyton to spatial differences in habitat hydraulics in streams is highly complex and it may be difficult to define clear hydr aulic habitat preference curves for periphyton communities in nutrient -poor streams.