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Trends in nuisance periphyton cover at New Zealand National River Water Quality Network sites 1990-2006 Online version
1. Introduction
Periphyton is algae that grows on the beds of streams and lakes. It plays a key role in streams by turning dissolved nutrients into nutritious food (i.e., periphyton biomass) for invertebrates, which are themselves food for fish and birds. However, there can be too much of a good thing. Periphyton blooms, as long filamentous growths or thick mats (Fig. 1) that cover much of the streambed, can make the stream unattractive for swimming and useless for angling, clog up water intakes, and reduce biodiversity by making the streambed habitat unsuitable for many sensitive invertebrate species (Biggs 2000). The Ministry for the Environment’s (MfE) guidelines for threshold periphyton cover levels to protect against aesthetic and recreational nuisances in streams and rivers are <30% of the visible bed for filamentous algae and <60% for mats (Biggs 2000). Cyanobacteria (commonly called “blue-green algae”) can cause off-flavours in water and fish flesh, foul-odours when they dry on stream margins, and may produce toxins (Biggs 2000).
Figure 1: Periphyton cover as filamentous green algae (left, photo J Quinn) and a thick diatom mat (right, photo from Biggs 1990).
Photographs showing filamentous green algae cover on a stream bed (left) and thick diatom mat on single stone (right).
Periphyton growth is controlled by sunlight reaching the streambed, time since the last scouring flow, streambed stability, current velocity, nutrients (mainly nitrogen and phosphorus), temperature and grazing by invertebrates (Biggs 2000). Healthy streams typically have little obvious periphyton, because growth is cropped by invertebrate grazers and turned into invertebrate biomass. Nuisance blooms are usually a symptom of a system stressed by factors like over-supply of nutrients and high temperatures (that increase algal growth rates and stress some invertebrate grazers). Therefore periphyton abundance can also be used as a measure of river health.
Periphyton monitoring in the National River Water Quality Network (NRWQN) was designed to answer the question: are periphyton growths that may cause an aesthetic nuisance becoming more frequent and/or worse over time? (Smith et al 1989). In this report, we focus on changes associated with land use and effluent discharge pressures and have excluded the effects of the stalked diatom bioinvader Didymosphenia geminata (commonly known as didymo) that appeared in the South Island late in the monitoring period (see Methods section for details). Periphyton was evaluated visually as cover by filamentous algae and mats (>2 mm thick, to distinguish from thin biofilms1) during monthly site visits whenever flow conditions allowed safe wading. Observations at 73 of the 77 NRWQN sites (Fig. 2) commenced in March – April 1989, and this report provides an analysis of these data collected for the 17-year period starting January 1990 and ending December 2006.
Figure 2: Locations of the 77 sites in the National Rivers Water Quality Network (NRWQN)
North and South Island maps showing the location of the 77 sites in the National River Water Quality Network.
Smith et al. (1989) classified the NRWQN sites as nominally baseline (“likely to be no or little effect of diffuse or point source pollution and which will account for natural or near-natural effects or trends”), impact (“downstream of present or possible future areas of agriculture, forestation, industry and urbanisation”) and pseudo-impact (not defined, but intermediate between the above).
1.1 Periphyton guidelines used
Periphyton cover at each site was assessed in relation to the MfE guidelines to protect against aesthetic and recreational values of rivers, which are less than 60% cover of the visible streambed by mats and less than 30% cover by filamentous algae (Biggs 2000). Also because rivers often had a mix of mats and filamentous growths, we also adopted 40% cover as the maximum acceptable level of total periphyton (i.e., mats + filamentous periphyton growths). This threshold was based on Biggs’ (2000) guidelines but weighted for the greater effect filamentous growths have on aesthetic value.
1 Biggs (2000) used >3 mm thickness to distinguish mats from thin biofilms (c.f. >2 mm in Smith et al. 1989), but this is unlikely to result in marked differences in visual assessment of mat cover.
