By John Quinn
As thin films and short/sparse filaments – the good Dr Jekyll – it is an important fuel at the base of healthy aquatic food webs, turning nutrients and sunlight into a food for invertebrates, which are in turn food for fish and waterbirds. In the process, they reduce the nutrient levels downstream or in a lake, helping to control eutrophication. Ecologist Maurice Lock once described periphyton as the “light and energy transducer of streams”. When viewed under a microscope, the species are also stunningly beautiful.
But its Mr Hyde side comes to the fore when periphyton forms abundant blooms that can degrade the looks, smell and taste of waterbodies, clog streambeds, and cause extreme fluctuations in pH and dissolved oxygen that harm sensitive aquatic organisms. When thick periphyton sloughs off the bed it can foul anglers’ lines, clog water intakes and sully the view for drift-feeding fish and humans alike. What’s more, the dark mats dominated by cyanobacteria, like Phormidium, can produce toxins that have been responsible for many dog deaths over the last decade.
It’s not surprising then that periphyton is one of the key river attributes assigned quality bands and bottom lines in the government’s recent National Objectives Framework proposals.
Research in NZ and overseas over the last couple of decades has led to guidelines for the levels of periphyton that separate the Dr Jekyll from Mr Hyde states, as both percentage cover/type and as biomass, in relation to different river values [PDF]. Refining these nuisance levels is an area of on-going research.
So what keeps periphyton in the desirable Dr Jekyll state? That comes down to the balance of growth-stimulating factors – mainly light, nutrients (nitrogen and phosphorus), temperature, steady flows and stable bed attachment sites – and controlling factors –high levels of flow disturbance and grazing by insects.
Conditions in summer are usually most conducive to periphyton blooms – long days and lots of sun; warm water increases algal growth rates and inhibits grazing; and the periods extend between spates (moderate flow spikes) that slough or scour the periphyton. So unfortunately, periphyton often blooms at the very time when people most want to use rivers for recreation and abstraction for irrigation, making them more of a nuisance.
Periphyton cover at National Rivers Water Quality Network sites is correlated with the percentage of pasture land use upstream, likely reflecting high nutrient inputs and light. Other research has shown periphyton increases below some point source wastewater discharges, after logging (unless riparian shade is retained), and in unshaded streams after urbanisation. On the other hand, maintenance and restoration of shade has been shown to reduce periphyton biomass.
So what can we do to manage periphyton in the desirable Dr Jekyll state? The main options are:
- Provide shade by enhancing riparian vegetation – this is most effective in small-medium streams with channels narrower than about 6 m, although trees with sprawling growth form can provide patches of heavy shade along rivers with wide channels where the main flow often hugs the banks.
- Control nutrient inputs – limiting the inputs of nitrogen and phosphorus so that concentrations are well below the optimal levels for periphyton development reduces biomass and/or the downstream extent of nuisance periphyton below inputs.
- Reduce fine sediment input –fines deposited during high flows, or due to insidious inputs, can be a source of phosphorus on the bed right where the periphyton need it. Fine sediment can also reduce grazer abundance by clogging their bed habitat and reducing the food quality of the periphyton.
- Manage stressors on grazing invertebrates – keep levels of toxicants like pesticides and ammonium well below stressful levels to maintain grazing rates that crop the periphyton
- Maintain scouring flows – avoid alterations to flow regimes that extend the periods for periphyton biomass build-up between spates that scour growths (typically flow increases to more than 3 times normal)
- Avoid spreading particular nuisance periphyton types like Didymo that are adapted to exploit low nutrient environments by following the Check/Clean/Dry protocols.
Dr John Quinn is Principal Scientist for Freshwater Ecology at NIWA.