Seasonality of macrophytes and interaction with flow in a New Zealand lowland stream

Abstract

Introduced submerged macrophytes have come to dominate many shallow water bodies in New Zealand, and are a common component of many lowland streams. We investigated the seasonal variation of macrophyte abundance, its influence on flow and channel volume, and the implications of this on stream habitat and functioning in Whakapipi Stream, a typical lowland stream draining a predominantly agricultural catchment.

Abundance of macrophytes over the summer was primarily controlled by the phenological cycles of the two dominant species. Mean minimum total macrophyte biomass (36 g m⁻²) and cover (7%) occurred in winter (June and August, respectively), and mean maximum biomass (324 g m⁻²), and cover (79%) occurred in late summer (March and February respectively). Egeria densa comprised the majority of both cover and biomass during the study period, except early summer (December) when Potamogeton crispus was prevalent in the shallow stream reaches.

Macrophyte beds had a major impact on summer stream velocities, reducing average velocities by an estimated 41%. Stream cross-sectional area was maintained at relatively stable levels similar to that recorded over winter, when stream discharge was in the order of seven times greater. The mean velocity distribution coefficient (α), and Manning's roughness coefficient (n) were dependent on and displayed a positive linear relationship with macrophyte abundance. The velocity distribution coefficient is recommended as a better indicator of macrophyte effects on velocity in natural streams, as it does not assume uniform velocity, channel depth and slope within the stream reach.

Our study shows that submerged macrophytes play an important structuring role within the stream during the summer period, where macrophyte beds act as semi-permeable dams, retarding flow velocities and increasing stream depth and cross-sectional area. This promotes habitat heterogeneity by creating a greater range of flow velocity variation, and also provides large stable low-flow areas. Other likely ecosystem effects resulting from macrophyte/velocity interactions include increased sedimentation, potential for nutrient processing and increased primary production, both by macrophytes and attached epiphyton. The complex architecture of submerged macrophytes and their influence on stream flow may also provide an increased diversity of habitat for other aquatic biota. We propose that management of degraded lowland streams such as the Whakapipi Stream to maintain stretches with moderate quantities of submerged macrophytes interspersed with shaded areas would optimise stream health during low summer flows.