The effects of low summer discharge on salmonid ecosystems

Abstract

The chalk streams of England are predominantly groundwater fed and, as a consequence, have a high base flow index. Increasing water demand and resulting abstraction from the groundwater aquifers that support chalk stream flows, coupled with reduced recharge of aquifers as a consequence of projected climate change, are among the biggest threats to the condition of chalk stream ecosystems. Despite this, the ecological implications of the potential changes in river discharge have received limited attention, at a river level. This PhD research used a large(stream)-scale discharge manipulation experiment in three chalk streams within the River Itchen catchment (Hampshire, UK), where sluice gates at the top of each stream enabled complete control of discharge, to investigate the ecosystem level response to simulated drought (reduced summer discharge). Experimental summer discharge reductions of 50% and 90% were selected based on long-term records of summer discharge (1975 - 2018) on the River Itchen and River Test, and implemented on each of the three streams over three consecutive years using a temporal block design. Physical characteristics, basal resources and macroinvertebrates in the streams were monitored, as well as the diet, habitat use, growth, movement and population size of the salmonids present. Sampling occurred before, during and after a 30-day long reduction in discharge each summer. Changes in the physical habitat were quantified by repeated recordings of water depth, velocity, wetted width and temperature, and samples of basal resources (FPOM, CPOM, benthic algae) were taken. The response of macroinvertebrates and prey availability for salmonids was determined by collecting Surber and drift net samples. Salmonid diet was quantified by analysing stomach contents and salmonid movements were monitored using Passive Integrated Transponder (PIT) tag technology. Salmonid populations were monitored using electrofishing. Despite substantial reductions in water depth, velocity and wetted width, and an increase in mean and variation of water temperature, there were limited changes in basal resources and no effect on macroinvertebrate density as a result of 3 discharge reduction. Reduced discharge resulted in a significant change in macroinvertebrate community composition, but the size of the effect was small in comparison with the variation between sampling occasions (seasonal response). In addition to a limited response by invertebrates, salmonids displayed high dietary plasticity. For example, 0+ trout consumed larger prey items within the discharge reduction treatments compared with the control. Site loyalty decreased for salmon, 0+ and =2+ trout during the 90% discharge reduction. Older (=2+) trout were more likely to move out of the affected area during a 90% discharge reduction, which corresponded with reduced site loyalty. Salmon were the only species/cohort to move back into the study area after the reinstatement of pre-manipulation discharge, potentially due to reduced competition by older (=2+) trout. There were no lasting effects of discharge reduction on site loyalty, which indicates that these salmonids were resilient to reduced discharge conditions. Yearling (1+) trout adopted a 'sit it out' strategy during reduced discharge conditions. Adopting this strategy increased growth rate and allowed for the expansion of area used once discharge was reinstated to premanipulation levels. There were no effects of discharge reduction on population size, although there was a slight (but not significant) effect on salmonid population density after the streams had experienced a 90% discharge reduction. In addition, differences amongst streams could not explain any additional variation within the salmonid data. This research highlights that, despite a marked response in the recorded physical characteristics of the streams, macroinvertebrates and salmonids within these chalk streams display a remarkable resistance/resilience to short term summer discharge reduction. This suggests that they are highly adaptable species and during short-term summer discharge reduction it may be better for river managers to not intervene, even under severe discharge reductions.