Our study suggests that the nutrient response is suppressed by other controlling factors on the lower limb of the nutrient response curve (i.e. at low nutrient concentrations); a quantitatively significant response occurs only in excess of a threshold beyond which nutrients become dominant over other controlling factors. This interpretation of the results is consistent with published meta-analyses showing lack of nutrient response for a high proportion of experimentally enriched periphyton communities, and division of responses between N and P for communities that do show growth in response to enrichment.

Periphyton biomass rarely responded positively to in-situ experimental enrichment with nitrogen or phosphorus. In the summer, nutrient enrichment overall had no effect on periphyton biomass, while outside the growing season N enrichment had inhibitory effects on periphyton.

4. Despite these experimental results, surveys of ambient chlorophyll a concentrations in streams across the HBEF demonstrated no relationship between streamwater dissolved inorganic N or P concentrations and benthic chlorophyll a.

5. Our results suggest that HBEF periphyton communities are not closely regulated by nutrient availability, even during periods of high light availability. The inhibitory effects of nutrient enrichment outside the growing season are interesting, but further research is necessary to elucidate the mechanisms driving these responses

We found that if algae showed significant response to nutrient addition, N limitation (either N alone or N with P) was the most frequent response both on GF/F filters and on wood. Despite the low dissolved nutrient concentrations in our study streams, more than a third of the streams did not show any response to N or P addition

Our results indicate that in headwater streams with intact tree canopies, chronic nutrient enrichment at moderate concentrations may have little detectable effect on benthic algal composition or periphyton biomass. Although nutrients stimulated algal growth rates, the long-term effects of nutrient addition on periphyton biomass were small in magnitude compared with other published values and were potentially suppressed by light availability and invertebrate consumption

established Phormidium-dominated
mats are likely to be able to persist under lower than predicted DIN
concentrations due to nutrient cycling occurring, aided by nitrogen
fixing bacteria. The synthesis of these field-based studies suggests
that Phormidium proliferations can occur, or at least initiate, at low
ambient concentrations of both DRP and DIN. The ability to
accumulate biomass at low ambient nutrient concentrations and
accumulate and retain nutrients as they grow is a well described,
microbial mat trait (Bonilla et al., 2005; Quesada et al., 2008).

The greatest portion of variance in models for the mean and maximum biomass of benthic stream algae (about 40%) was explained by concentrations of total N and P. Breakpoint regression and a two-dimensional Kolmogorov–Smirnov statistical technique established significant breakpoints of about 30 µg total P·L–1 and 40 µg total N·L–1, above which mean chlorophyll values were substantially higher.

Meta-analysis of lotic nutrient (N and P) amendment experiments indicated that simultaneous stimulation of benthic algal community biomass by .1 nutrient was the rule,
not the exception. Addition of a limiting nutrient typically doubled algal biomass, whereas addition of another nutrient generally increased algal biomass ;1.25-fold. N was approximately equally likely as P to be limiting.

Only when reference nutrient concentrations are in the upper one third of those expected in the United States, is maximum benthic chlorophyll projected to exceed 100 mg m-2 (a concentration commonly used to indicate nuisance levels) >30% of the time

Analysis of the Maitai River
data showed that a reduction in DIN was not associated with a
reduction in Phormidium cover (Wood et al., 2015a).

Additionally, fine sediment might become entrained in high flows and scour algal biomass during floods (Schofield and others 2004).

4.RDAs showed that industrial organic compounds, herbicides and PhC products were the pollutants most strongly associated with measures of biofilm structure and function, whereas dissolved inorganic nitrogen, dissolved organic carbon and hydrological variability were the environmental factors most strongly associated with biofilm responses

Algal biomass is usually related to nutrient concentrations, and this was the case for epilithic algae on cobbles in our streams. However, other factors, including substrate size and stability, hydrology, and invertebrate grazing, affect algal and moss biomass in streams (Biggs, 1996; Biggs and Close, 1989; Suren and Duncan, 1999).

For river periphyton in general, Francoeur et al. (1999) found using artificial substrate experiments, that temperature was the most important variable affecting accrual rate. Heath et al. (2011) also suggested that water temperature is an important factor in determining whether Phormidium proliferations were present or absent, based on field observations of greater proliferation during warm, summer months

variability in algal biomass in this study was not related to in-stream concentrations of TIN
However, neither TIN nor SRP were low enough at our study sites to be considered limiting
Postulated that temperature was more important driver of periphyton biomass

Increasing nutrients by 60–99% of ambient concentrations increased periphyton percentage cover and biomass. Periphyton abundance also increased with increasing duration of exposure to nutrients (2, 4 and 8 weeks); however, short-term pulses of nutrients (2 weeks) had no significant effect in the rural stream. These results indicate that effective management of nutrient delivery, by reducing time periods of high nutrient load, will minimise impacts to benthic environments.

A literature review indicates (1) stream benthic chlorophyll is significantly correlated to both total N and total P in the water column, with both nutrients explaining more variance than either considered alone; (2) nutrients have increased substantially in many rivers and streams of the United States over reference conditions, and strong shifts in N and P stoichiometry have occurred as well; (3) bioassays often indicate N responses alone or in concert with P responses for autotrophic (primary production and chlorophyll) and heterotrophic (respiration) responses; (4) both heterotrophic and autotrophic processes are influenced by the availability of N and P; and (5) N-fixing cyanobacteria usually do not seem to be able to fully satisfy N limitations in rivers and streams when P is present in excess of N. These data suggest both N and P control should be considered in the eutrophication management of streams.