The effects of macrophyte control on freshwater fish communities and water quality in New Zealand streams

Abstract

The ecological effects of macrophyte removal on New Zealand's stream ecosystems are not well understood. To address this issue, I investigated how native fish abundance and diversity are impacted by mechanical excavation of macrophytes, and examined the role suspended sediment and dissolved oxygen play in driving changes in community structure following macrophyte removal. Population surveys conducted before and after experimental mechanical excavation of macrophytes demonstrated that native fish abundance was reduced by 52 percent after excavation, but species diversity was not affected. Although partial macrophyte removal was still found to reduce fish abundance significantly, radiotelemetry of giant kokopu (Galaxias argenteus) demonstrated that this technique might prevent large individuals of this species from leaving targeted waterways.

Suspended sediment was monitored before and after a large macrophyte removal operation (> 80 kilometres of waterway excavated) by water sampling and continual turbidity measurements. Immediate and dramatic increases in suspended sediment concentrations were observed during and immediately after mechanical excavation of macrophytes (120,000 % increase). Suspended sediment concentrations remained elevated for 77 days after macrophyte removal, and were particularly high during periods of flooding. Sediment concentrations regularly exceeded concentrations required to elicit an avoidance response in juvenile migratory native fish and introduced salmonids, suggesting fish abundance may be reduced. Respirometry trials demonstrated that suspended sediment concentrations recorded after macrophyte removal had no effect on the respiratory performance of brown trout (Salmo trutta). However, laboratory-based feeding experiments demonstrated that the same sediment concentrations can reduce the feeding rates of this species by up to 43 percent. In addition, suspended sediment can affect other aspects of water quality to the extent that fish may be affected. Dissolved oxygen concentrations were continually monitored in several streams following mechanical excavation of macrophytes and herbicide application. Statistically detectable reductions in dissolved oxygen concentration, associated with the resuspension of highly organic anoxic sediment, were observed immediately after mechanical excavation of macrophytes. Time spent in moderate (the daily percentage of DO measurements below 30 percent saturation) and severe hypoxia (the daily percentage of DO measurements below 30 percent saturation) was 43 percent and 37 percent greater in the three-day period after mechanical excavation than in the three-day period before. At a number of sites included in this study hypoxia persisted for a several days, and previous research suggests this would have been sufficient to cause significant mortality in the resident fish population. Periods of hypoxia associated with plant decomposition were also observed following herbicide application. However, reductions in dissolved oxygen were gradual, and it is expected that fish were able to move out of treated waterways before oxygen conditions became lethal. The results of this study indicate that mechanical macrophyte removal is likely to cause significant adverse impacts on fish communities in New Zealand waterways. Furthermore, increased sediment resuspension and associated changes in dissolved oxygen concentration after macrophyte removal may have a greater impact on resident fish than previously thought.