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Long-term declines in phosphorus export from forested catchments in south-central Ontario M. Catherine Eimers, Shaun A. Watmough, Andrew M. Paterson, Peter J. Dillon, and Huaxia Yao
Abstract: Total phosphorus (TP) levels in many Canadian Shield lakes in central Ontario have declined over recent decades, despite increases in human activity in most watersheds. To investigate the contribution of changes in catchment export to long-term declines in lake TP, we examined temporal and spatial patterns in TP concentrations and export (1980–1981 to 2001–2002) across 11 subcatchments that drain into three lakes in which average ice-free TP levels have declined by approximately 35%. Annual stream export of TP decreased significantly by 30%–89% in eight of the 11 subcatchments, and decreases in export were driven by declines in TP concentration, not changes in stream flow. Annual average TP concentrations varied fivefold among adjacent subcatchments, and temporal patterns in annual average TP concentrations were poorly correlated. Seasonal patterns of TP concentration were most similar among streams in the spring (March–April–May), and TP export in the spring declined significantly in 10 of the 11 subcatchments. Because spring melt is the principal hydrologic event in these seasonally snow-covered basins, decreases in TP export during the spring were primarily responsible for declines observed in annual export. The drivers of changes in TP over time are unclear at this point but are the focus of current research. Re´sume´ : Les concentrations de phosphore total (TP) dans plusieurs lacs du Bouclier canadien du centre de l’Ontario ont diminue´ au cours des dernie`res de´cennies, malgre´ l’augmentation des activite´s humaines dans la plupart des bassins versants. Afin d’e´tudier la contribution des changements dans l’exportation de TP des bassins versants aux de´clins a` long terme du TP lacustre, nous examinons les patrons temporels et spatiaux de concentrations et d’exportation de TP (1980– 1981 a` 2001–2002) dans 11 sous-bassins qui se de´versent dans trois lacs dans lesquels les concentrations de TP durant la pe´riode sans glace ont chute´ d’environ 35 %. L’exportation annuelle de TP dans les cours d’eau a de´cru significativement de 30–89 % dans huit des 11 sous-bassins et les de´clins de l’exportation de TP s’expliquent par des diminutions des concentrations de TP et non par des changements de de´bit des cours d’eau. Les concentrations annuelles moyennes de TP varient par un facteur de cinq dans des sous-bassins adjacents et les patrons temporels des concentrations annuelles moyennes sont faiblement corre´le´s. Les patrons saisonniers des concentrations de TP entre les cours d’eau ont la similarite´ la plus forte au printemps (mars–avril–mai) et l’exportation de TP au printemps a diminue´ significativement dans 10 des 11 sousbassins versants. Parce que le de´gel printanier est l’e´ve´nement hydrologique principal dans ces bassins versants couverts de neige en saison, la diminution de l’exportation de TP au printemps est la cause principale des de´clins observe´s dans l’exportation annuelle. Le ou les facteurs explicatifs des changements de TP au cours du temps restent obscurs, mais font actuellement l’objet de recherches. [Traduit par la Re´daction]
Introduction Declining total phosphorus (TP) concentrations have been reported recently in a number of oligotrophic Canadian Shield lakes in south-central Ontario that are remote from Received 28 October 2008. Accepted 20 April 2009. Published on the NRC Research Press Web site at cjfas.nrc.ca on 26 September 2009. J20844 Paper handled by Associate Editor Yves Prairie. M.C. Eimers.1 Department of Geography, Trent University, Peterborough, ON K9J 7B8, Canada. S.A. Watmough and P.J. Dillon. Environmental and Resource Science Program, Trent University, Peterborough, ON K9J 7B8, Canada. A.M. Paterson and H. Yao. Ontario Ministry of Environment, Dorset Environmental Science Centre, 1026 Bellwood Acres Road, Dorset, ON P0A 1E0, Canada. 1Corresponding
author (e-mail:
[email protected]).
Can. J. Fish. Aquat. Sci. 66: 1682–1692 (2009)
agricultural, industrial, and urban sources of P (Quinlan et al. 2008; Yan et al. 2008). These declines in TP levels in lakes have occurred despite general increases in human activity (e.g., cottage usage) in their watersheds, and recent regional lake water chemistry surveys indicate that TP levels have decreased over a much broader region by at least 30% over the past three decades (A. Paterson, unpublished data). Decreasing TP concentrations in oligotrophic systems are of particular concern because of the relationship between P availability and aquatic productivity (Ho¨rnstro¨m 1999; Jeppesen et al. 2005) and shifts in aquatic community composition, such as an increase in the abundance of ‘‘taste and odour’’ algae, may occur in tandem with long-term declines in TP (Paterson et al. 2004). Declines in lake TP concentrations could be a result of internal (i.e., within-lake) and (or) external (catchment) factors. Internal processes that could influence lake TP include lake water acidification (and changes in sediment organic matter mineralization and (or) Al complexation), changes in internal loading (e.g., changes in the reducing conditions in
doi:10.1139/F09-101
Published by NRC Research Press
Eimers et al.
bottom sediments), biological changes (e.g., ecological shifts), and hydroclimatic changes (e.g., increases in water residence time, duration of the ice-free season, or stratification). However, because TP declines have been observed in a number of lakes across a relatively large area, including lakes with and without shoreline residential development, we hypothesize that the causal factors must operate on a regional scale and are not lake-specific. Furthermore, we are specifically interested in whether changes in external loading, i.e., declines in TP loads from the catchment and (or) the atmosphere, can account for declines in lake TP, and as such, we focus our investigation on catchment controls. The export of TP from catchments to lakes is affected by a number of factors, many of which may change over time, including atmospheric deposition (phosphorus and acids), weathering, hydrology, climate, and land use. Changes in atmospheric deposition of P have an obvious direct impact on P inputs to lakes, and atmospheric inputs of P may be a large fraction of the P budget of lakes with relatively large surface-area to catchment-area ratios (Dillon et al. 1993) and where weathering fluxes are low (Jansson et al. 1986; Doskey and Ugoagwu 1989). Acid deposition may also affect lake TP levels, as elevated Al transport from acidified soils could result in the inactivation of P in lakes (Huser and Rydin 2005; Kopacek et al. 2007). Moreover, acidification of catchment soils could result in stronger adsorption of TP to soil mineral surfaces and lower leaching rates (e.g., McDowell et al. 2002). Wetlands and riparian areas are particularly important loci of P cycling in undisturbed catchments, as they are a major source of dissolved organic matter (including dissolved organic P) in streams. In addition, redox processes within wetlands can alter stream P levels by affecting the solubility of iron–phosphorus (Fe–P) compounds, and Fe and TP are often correlated (increasing under reducing conditions; decreasing under oxic conditions) in wetland outflows (Devito and Dillon 1993a, 1993b; Carlyle and Hill 2001). Changes in land use, including wetland creation–degradation and forestry (reforestation or harvesting), can therefore increase or decrease P delivery to lakes by influencing the sources and pathways of P inputs (e.g., Hall and Smol 1996). Changes in stream flow may also impact the delivery of P to lakes. Because of the tight biological cycling of P, the majority of P export from forested catchments occurs during relatively brief periods of high runoff, including storms and snowmelt (Meyer and Likens 1979). As such, changes in the magnitude and timing of high discharge events could influence P delivery to lakes. Hydroclimatic shifts could also influence the amount of labile P available for leaching by altering the rate of P uptake by biota or release from soils through mineralization. It is clear that a variety of external factors could contribute to declining lake TP concentrations, and the relative importance of each factor may differ among catchments with different landscape characteristics (i.e., wetlands, slope, geology, etc.). To investigate the role of changes in the delivery of P to lakes, we examined long-term (1980–1981 to 2001–2002) trends in TP deposition and export from 11 streams that drain into three lakes in which TP levels have declined over the past two decades. The three lakes (Harp, Plastic, and Dickie) are located within 40 km of one an-
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other, are remote from point sources of TP deposition, and are not impacted by agriculture or urbanization. Plastic Lake is surrounded by intact coniferous forest, and there is no cottage development within its watershed. In contrast, Harp and Dickie lakes have substantial cottage populations (Harp, 110 residences in 2004; Dickie, 143 residences in 2001) and are surrounded by mixed hardwood forest and selective cutting by landowners has occurred in some of their subcatchments. All three watersheds receive similar inputs of bulk acidic and phosphorus deposition. Specific objectives of this study are (i) to identify whether TP inputs to lakes in stream export and bulk deposition have changed over time, (ii) identify whether coherent seasonal changes in TP concentrations and export have occurred across the 11 study streams, (iii) examine relationships between TP and other chemical parameters to test for possible drivers of changes in TP, and (iv) determine whether changes in external inputs of TP to lakes can account for observed declines in lake TP.
Materials and methods Study area The 11 headwater subcatchments (HP3, HP3A, HP4, HP5, HP6, HP6A, DE5, DE6, DE8, DE10, PC1; Table 1) considered for this study are located in the District Municipality of Muskoka (Harp subcatchments, HP; Dickie subcatchments, DE) or Haliburton County (Plastic Lake subcatchment, PC) and are part of the Ontario Ministry of Environment Dorset Environmental Science Centre’s (DESC) long-term monitoring program, which has been in operation since the mid- to late 1970s. The study area is located at the southern edge of the Canadian Shield and is typical of the shallow till – rock ridge physiographic region of south-central Ontario (Chapman and Putnam 1984). Plastic Lake is a small (32 ha; mean depth 7.9 m) headwater lake in an area of granitic gneiss bedrock overlain by a thin discontinuous layer of sandy basal till. Seven streams drain into Plastic Lake, six of which are ephemeral, draining a total watershed area of 92 ha. The largest inflow (Plastic Lake 1 (PC1); 23 ha) flows almost continuously (ceasing to flow only during particularly dry summers) and has been monitored for stream flow and chemistry using consistent methods since 1980. Plastic Lake has an average residence time (1980– 2001) of 4.1 years. Upland soils at PC1 are generally thin (80% sand,
