Hydrologic and hydraulic control of macrophyte establishment and performance in streams

Macrophytes play a key role in many unshaded lotic ecosystems, but little is known of the factors controlling their presence, abundance, and composition. Macrophyte abundance, diversity, and composition were studied in 15 New Zealand streams to test the hypotheses that the presence and development of macrophytes in lotic systems is primarily controlled by the hydrologic regime (frequency of high-velocity flood events) and that the interflood spatial distribution and performance of taxa in more stable systems is strongly influenced by local hydraulic conditions (depth/velocity/sediments). Both hypotheses were supported by our results. We found that the abundance and diversity of macrophytes decreased as flood disturbance frequency increased (r 5 0.52, P 5 0.002 for abundance; r 5 0.53, P 5 0.022 for diversity) and that vegetation was absent in streams with more than ;13 highflow disturbances per year. An experiment in an ecohydraulics flume identified that the main mechanism causing these effects was not stem breakage at high water velocity but probably uprooting associated with bed sediment erosion. We found that plants with high propagule production constituted a greater proportion of the vegetation in more flood disturbed streams than in stable streams, suggesting that this species trait is important for the maintenance of macrophyte communities in flood prone streams. Distinct velocity, depth, and substrate particle size habitat preferences were displayed by four common species in the study streams. None of the macrophytes showed overlapping preferences for all three habitat variables, suggesting coexisting of the species in streams by physical niche separation. These results significantly expand our understanding of the role of flow regimes in determining lotic ecosystem structure and functioning. Macrophytes play a key role in unshaded streams by increasing physical heterogeneity, trapping fine sediments, and providing extensive habitat for periphyton, invertebrates, and fish (Biggs 1996a). However, macrophytes can also proliferate and severely impede water flow, degrade water quality through their effects on pH and dissolved oxygen, and degrade aesthetic/recreational values in streams (Haslam 1978; Nichols and Shaw 1986; Biggs 1996a). Optimal management of streams will require information to predict macrophyte abundance and diversity. At present, we cannot even answer basic questions such as why macrophytes colonize and grow successfully in some streams but not others, and once they do colonize what controls patchiness, overall biomass, and community structure. Obtaining such knowledge is very important because stream macrophytes can drive physical conditions, periphyton, benthic invertebrates, and (possibly) fish communities to quite different states compared with unvegetated channels (e.g., Burkholder 1996; Death 2000). Biggs (1996a) posited a hierarchical conceptual model of factors that may strongly influence macrophyte development 1 Current address: Department of Plant Ecology, University of Aarhus, Nordlandsvej 68, DK-8240 Risskov, Denmark (tenna. [email protected]).