McDowell et al.—Eutrophication of surface waters 985 New Zealand Journal of Marine and Freshwater Research, 2009, Vol. 43: 985–995 1175-8805 (Online); 0028–8330 (Print)/09/4304–0985 © The Royal Society of New Zealand 2009
Nitrogen and phosphorus in New Zealand streams and rivers: control and impact of eutrophication and the influence of land management R. W. McDowell1 S. T. Larned2 D. J. Houlbrooke1 1 AgResearch Invermay Agricultural Centre Private Bag 50034 Mosgiel 9011, New Zealand email:
[email protected]
National Institute of Water and Atmospheric Research Limited P.O. Box 8602 Riccarton Christchurch, New Zealand
2
Abstract Given sufficient light and heat, the growth of aquatic macrophytes and algae associated with eutrophication is generally controlled by the concentration, form and ratio between nitrogen (N) and phosphorus (P). Data from 1100 freshwater sites monitored for the last 10 years by New Zealand’s regional councils and unitary authorities were assessed for streams and rivers with mean nitrate/ nitrite-N (NNN), dissolved reactive P (DRP), total N (TN) and total P (TP) concentrations in excess of New Zealand guidelines, and to generate a data set of N:P ratios to predict potential periphyton response according to the concentration of the limiting nutrient. The frequency of sites exceeding the guidelines varied from 0 to 100% depending on the parameter and region, but South Island regions were generally more compliant. The dissolved inorganic N (DIN) to dissolved reactive P (DRP) ratio was used to group data into three nutrient limitation classes: 15:1 (P-limited), by mass. P-limitation was the most frequent scenario in New Zealand streams (overall, 76% of sites were P-limited, 12% N-limited, and 12% co-limited). M09003; Online publication date 17 August 2009 Received 26 January 2009; accepted 9 April 2009
The mean concentration of the limiting nutrient for each site was combined with empirical relationships to predict periphyton densities (the average of Nand P-limited growth was used for sites with colimitation). This assessment predicted that 22 sites were likely to exceed the periphyton guideline for protecting benthic biodiversity (50 mg chlorophyll a m–2), but this assessment is likely to be highly changeable in response to climatic conditions and present and future land use. As an example, we modelled N and P losses from an average sheep and a dairy farm in Southland (South Island, New Zealand) in 1958, 1988, 2008 and 2028. We predicted that with time, as farm systems have and continue to intensify, N losses increase at a greater rate than P losses. Since the pathway for N to reach fresh waters may be more tortuous and take longer than P to reach a stream or river, focusing mitigation on P losses may have a quicker effect on potential algal growth. In addition, with time, it is expected that P-limitation in New Zealand’s rivers and streams will be more widespread as N-losses are unabated. Hence, although strategies to decrease N losses should be practised, mitigating P losses is also central to preventing eutrophication. Keywords algae; dairy; dissolved; land use; periphyton; sheep Introduction Nutrient supply is a controlling factor in the pro liferation of aquatic macrophytes and algae associated with eutrophication of streams and rivers (Carpenter et al. 1998). Bioavailability of either nitrogen (N) or phosphorus (P) often limits macrophyte and algal growth rates (Schindler et al. 2008). Bioavailability can be split into three components: the relative abundance of N and P (indicated by N:P ratios), the concentrations of N and P, and the chemical forms of N or P (Wetzel 2001). Redfield et al. (1963) published data that indicated a molar ratio of carbon (C), N and P of 100:16:1 was
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reasonably constant during phytoplankton growth. This ratio is commonly used as a general indicator of plant and algal growth potential in marine and freshwater systems (Schindler et al. 2008). Generally, the supply of C is non-limiting, which means that if considerably more than 16 moles of N are present for each mole of P, then growth is predicted to be P-limited, and if considerably less than 16 moles of N are present for each mole of P, growth is predicted to be N-limited. On a mass basis, the Redfield N: P ratio is 7:1. Applications of the Redfield ratio can be confirmed by measuring N:P in plant and algal biomass and by bioassays, which are the “gold standard” for nutrient limitation (Francoeur et al. 1999). Such testing may indicate N or P limitation at ratios different to those indicated by the Redfield ratio owing to competition or to variable nutrient requirements among periphyton species (Klausmeier et al. 2004). However, bioassays only represent a point in time and often the use of a ratio derived from several samplings can be a useful indicator of the potential for N or P limitation when conditions for growth are favourable. Departures from Redfield-based predictions of growth limitation have been reported for streams and lakes in New Zealand (e.g., Francoeur et al. 1999; Biggs 2000a; Downs et al. 2008). Internationally, Guildford & Hecky (2000) confirmed the use of N:P ratios as a tool to predict nutrient limitation of algal growth in several lake and ocean sites, but the ratios were wider (20 (N:P) for P-limitation) than the Redfield ratio. Given this variation, the conservative approach is to use N:P ratios only as indicators of extreme N- or P-limitation, where limitation by the one nutrient is highly likely, and limitation by the other is highly unlikely. In this study, we used a compromise between the Redfield ratio and the ratios published by Guildford & Hecky (2000) of 15:1 for Plimitation (mass basis). These ratios were analogous to those used by the Ministry for the Environment (2007) and White (1983). The ratio of N:P in fresh waters needs also to be put in context relative to N and P concentrations. For instance, if concentrations of both N and P are high, periphyton blooms may occur even if one nutrient is limiting. Consequently, periphyton guidelines that are intended to prevent the impairment of benthic biodiversity or trout habitat can be exceeded even if one nutrient is limiting (e.g., Biggs et al. 2000a). The final component of bioavailability is physio chemical form. Both N and P exist in dissolved (or soluble) and particulate form. Dissolved fractions are
usually defined as those that pass through a 0.45 μm filter (McDowell et al. 2004). The distinction in bioavailability to algae is the relative speed, or kinetics, of dissolved P uptake. Dissolved inorganic forms of N and P are immediately available for uptake (including some organic forms), whereas nutrient ions on and within particles must be released through enzymatic or physical processes before they are available to plants and algae (McDowell et al. 2004). Hence, ratios of dissolved inorganic N (DIN; nitrate-N, nitrite-N + ammoniacal-N) to dissolved reactive P (DIN:DRP ratios) are used in fast-flowing streams and rivers (Biggs et al. 2000a). In contrast, in slow-flowing rivers and lakes, reservoirs, ponds, marshes with high residence times, particles settle out and release dissolved nutrients. Hence, total N to total P (TN:TP) ratios are commonly used for lentic systems (Chapra 1997). In this paper, we present a national analysis of N and P data from 1100 surface water quality sites sampled over the last 10 years by New Zealand regional councils and unitary authorities (collectively referred to as regions). To focus on the potential for nutrient limitation, we assumed that other controlling factors such as light, temperature and other macro- and micro-nutrients were non-limiting for periphyton growth. Our first objective was to present summary statistics for N to P concentrations and dissolved N:P ratios to determine which nutrient is limiting (or controlling) algal proliferation. Our second objective was to use dissolved N and P data to estimate periphyton biomass levels. Although the analyses focused on the relative importance of N or P in limiting periphyton growth, our intent was to also inform discussions about the effectiveness of best management practices (BMPs) for protecting and improving surface water quality. Land use in New Zealand has changed considerably over the past 50–100 years and these changes are likely to continue (MacLeod & Moller 2006). With land-use changes come changes in the relative losses of N and P to surface waters. Furthermore, the routes by which N and P are transported to surface waters mean that their impacts may vary. For example, N input to surface waters is usually dominated by slow flowing sub-surface or groundwater flow, whereas P input is usually dominated by faster overland or shallow sub-surface routes (Carpenter et al. 1998). The transport pathways mean that for N there can be a considerable time lag between activities on land, subsequent N losses to surface waters, and variation in the benefit of BMPs if this is lag is not considered. Hence, our third objective was to model
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Fig. 1 Locations and boundaries of unitary authorities in New Zealand.
some typical farming systems of the past, present and future, simulate the relative dissolved inorganic N and P loss rates, and comment on the implications for farm management to prevent excessive algal growth. Materials and methods Stream or lake water quality data from 1100 sampling sites around New Zealand were acquired from regional councils/unitary authorities (Fig. 1). Of these data, 98% were for streams and rivers and used here. The sampling period was 1996–2007, except for one region, Environment Southland, which supplied data for 1996–2002. Included sites met the following criteria: there was a minimum of 6 samples for DIN, DRP, TN and TP concentrations distributed over springsummer-autumn; and, after additional inspection to correct for data formatting, sites had 15:1), N-limitation (15,
