Climate factors probably govern broad-scale patterns in macrophyte species distributions. At a local scale, distribution and biomass of macrophytes tends to be highly spatially and temporally variable. Variability is due to things such as environmental variability, recovery time from disturbance, and the likelihood that plant propagules can successfully establish. Environmental factors include flow (particularly extreme events), water velocity, water depth (and therefore flow alteration/extraction), substrate composition, substrate nutrient concentrations, water quality, riparian shading, competition and herbivory (Mackay. and James 2016). As you point out, the processes dictating plant composition and abundance are complex. Biggs (1996) states that light, nutrients and temperature predominantly govern the processes of plant biomass gain and hydraulic factors (incorporating separate temporal and spatial scales) predominantly govern the processes of biomass loss. Quinn et al. 2009, in relation to integrated catchment management at Whatawhata, postulated that increase in NOX was driven in part by reduced instream uptake of N due to increased shading that reduced stream temperatures and instream plants biomass (i.e., that plant growth was higher before restoration but decreased due to shade, not nutrients (NOx became higher). In contrast, O’Brien et al. (2014) found that for 3 lowland enriched streams, macrophytes drive ecosystem metabolism but had limited influence on water column nutrient concentrations because macrophyte demand was much lower than the supply available from the water column. Champion and Tanner (2000) show how macrophyte abundance varies with season.
Internationally, Dodds and Welch (2000) state: “Criteria for setting nutrient levels in lotic ecosystems are relevant to US states and other countries in the process of setting water-quality regulations. There are few articles in the peer-reviewed literature on this topic, and policy makers have had little information from which to base their decisions for streams”. Camargo and Alonso (2006) show that increased N can increase primary producer growth, biomass, proliferation, production (phytoplankton, periphyton, macrophytes) and in particular, elevated concentrations of DRP and dissolved inorganic nitrogen (NO3-N and NH4-N) may induce
excessive growths of macrophytes that retard water flows. Like Biggs (1996), others internationally have found that water velocity and temperature are key factors driving macrophyte assemblages (Manolaki and Papastergiadou (2013).