Merrin L. Macrae

Phosphorus transport in agricultural subsurface drainage: a review.

Phosphorus (P) loss from agricultural fields and watersheds has been an important water quality issue for decades because of the critical role P plays in eutrophication. Historically, most research has focused on P losses by surface runoff and erosion because subsurface P losses were often deemed to be negligible. Perceptions of subsurface P transport, however, have evolved, and considerable work has been conducted to better understand the magnitude and importance of subsurface P transport and to identify practices and treatments that decrease subsurface P loads to surface waters. The objectives of this paper were (i) to critically review research on P transport in subsurface drainage, (ii) to determine factors that control P losses, and (iii) to identify gaps in the current scientific understanding of the role of subsurface drainage in P transport. Factors that affect subsurface P transport are discussed within the framework of intensively drained agricultural settings. These factors include soil characteristics (e.g., preferential flow, P sorption capacity, and redox conditions), drainage design (e.g., tile spacing, tile depth, and the installation of surface inlets), prevailing conditions and management (e.g., soil-test P levels, tillage, cropping system, and the source, rate, placement, and timing of P application), and hydrologic and climatic variables (e.g., baseflow, event flow, and seasonal differences). Structural, treatment, and management approaches to mitigate subsurface P transport-such as practices that disconnect flow pathways between surface soils and tile drains, drainage water management, in-stream or end-of-tile treatments, and ditch design and management-are also discussed. The review concludes by identifying gaps in the current understanding of P transport in subsurface drains and suggesting areas where future research is needed.

Observed and Projected Climate Change in the Churchill Region of the Hudson Bay Lowlands and Implications for Pond Sustainability

Abstract There is concern over the fate of surface water bodies at high latitudes as a consequence of rising global temperatures. The goal of this study is to characterize climatic change that has occurred in the northern Hudson Bay Lowlands (HBL), Canada, from 1943 to 2009, to determine if this has resulted in a change to pond surface areas and to predict if changes may continue in the future. Climate change and changes to pond volume and size over the past ∼60 years were examined using a combination of field methods/instrumental records (1943–2009), modeling (1953–2009; 1961–2100), and remote sensing/imagery analyses (1947–2008). Results demonstrate that temperatures are warming and breakup dates are earlier, but this has not significantly increased the duration of the open-water period or pond evaporation rates, which can be highly variable from year to year. Annual precipitation, primarily summer rainfall, has increased, lessening the summer moisture deficit and leading to wetter conditions. The observed changes of a smaller summer moisture deficit are predicted to continue in future, although there is less certainty with predictions of future precipitation than there is with predictions of air temperature. Thus, ponds are likely not at risk for drying and instead may be at risk for expansion. Despite the increases in summer rainfall, imagery analysis of 100 ponds shows that pond surface areas have fluctuated over the study period but have not increased in size.

Nutrient Uptake and Short-Term Responses of Phytoplankton and Benthic Algal Communities from a Subarctic Pond to Experimental Nutrient Enrichment in Microcosms

Abstract Climate warming is anticipated to affect high-latitude regions, including abundant ponds of the Hudson Bay Lowlands (HBL). However, it remains unclear if associated increased frequency of nutrient pulses will be rapidly consumed by aquatic biota and sediment or lead to a rise in pond-water nutrient concentrations. Here, we performed a nutrient-amendment experiment to examine short-term (≤72 h) nutrient uptake and identify the consumers of the added nutrients (planktonic vs. benthic communities). Microcosms (1 L) with and without sediment were experimentally amended with inorganic nitrogen (nitrate, ammonium) with and without phosphate. Amended nitrate and ammonium concentrations remained high in microcosms without sediments, and phytoplankton biomass did not change relative to the un-amended control. However, phosphate concentration declined significantly in microcosms without sediment, resulting in significant increase of phytoplankton biomass after 72 h. In the presence of sediment, amended nutrients were rapidly removed from the water, stimulating benthic algal biomass when phosphate was co-amended with ammonium or nitrate. Phytoplankton biomass was significantly elevated in microcosms with sediment compared to those without sediment, regardless of whether nutrients were amended or not, indicating that sediment and associated benthic biofilm stimulate phytoplankton growth, likely via supply of nutrients to the overlying water column. A key outcome of the experiment is that pulsed nutrients were taken up rapidly and primarily by the benthic community. Findings suggest that shallow ponds in the HBL are capable of rapidly consuming pulsed nutrient supplies, as may occur due to hydroclimatic events, climate warming and other disturbances.

Hydrological Connectivity and Basin Morphometry Influence Seasonal Water-Chemistry Variations in Tundra Ponds of the Northwestern Hudson Bay Lowlands

Abstract Due to shallow depth and high surface area—to—volume ratio, ponds of the Hudson Bay Lowlands are vulnerable to climatic and hydrological changes, but relations between hydrological processes and limnological conditions remain unknown. Here, we measured water balance and limnological variables (water chemistry, suspended sediments, chlorophyll-a) at 20 ponds near Churchill (Manitoba) three times during the ice-free season of 2010 to explore relations among hydrological connectivity, basin morphometry, and waterchemistry variations. Using principal components analysis, we identified that the ponds followed one of four distinctive “seasonal water chemistry trajectories” (SWCT1–4). Most of the ponds that lacked apparent hydrologic connectivity displayed SWCT1, characterized by rising alkalinity and ionic content between early June and late July due to evaporative concentration. In contrast, most ponds with apparent hydrological connectivity displayed SWCT2 or SWCT3, characterized by marked changes in suspended sediment and total nitrogen concentrations due to inflow that transferred allochthonous materials from the catchment. Ponds in SWCT2 likely possessed temporary hydrological connections during periods of relatively high water supply and exhibited marked decline of suspended sediment and total nitrogen content when hydrological connection was lost. Most ponds in SWCT3 maintained active hydrological connections during all or most of the ice-free season and possessed relatively high suspended sediment and total nitrogen concentrations throughout the season. Ponds in SWCT4 possessed relatively stable water chemistry due to greater water depth and local features that reduced wind-induced sediment resuspension. We conclude that hydrological connectivity and basin morphometry exert important influence on seasonal pond water-chemistry dynamics.

Limnological regime shifts caused by climate warming and Lesser Snow Goose population expansion in the western Hudson Bay Lowlands (Manitoba, Canada)

Shallow lakes are dominant features in subarctic and Arctic landscapes and are responsive to multiple stressors, which can lead to rapid changes in limnological regimes with consequences for aquatic resources. We address this theme in the coastal tundra region of Wapusk National Park, western Hudson Bay Lowlands (Canada), where climate has warmed during the past century and the Lesser Snow Goose (LSG; Chen caerulescens caerulescens) population has grown rapidly during the past ∽40 years. Integration of limnological and paleolimnological analyses documents profound responses of productivity, nutrient cycling, and aquatic habitat to warming at three ponds (“WAP 12”, “WAP 20”, and “WAP 21″), and to LSG disturbance at the two ponds located in an active nesting area (WAP 20, WAP 21). Based on multiparameter analysis of 210Pb-dated sediment records from all three ponds, a regime shift occurred between 1875 and 1900 CE marked by a transition from low productivity, turbid, and nutrient-poor conditions of the Little Ice Age to conditions of higher productivity, lower nitrogen availability, and the development of benthic biofilm habitat as a result of climate warming. Beginning in the mid-1970s, sediment records from WAP 20 and WAP 21 reveal a second regime shift characterized by accelerated productivity and increased nitrogen availability. Coupled with 3 years of limnological data, results suggest that increased productivity at WAP 20 and WAP 21 led to atmospheric CO2 invasion to meet algal photosynthetic demand. This limnological regime shift is attributed to an increase in the supply of catchment-derived nutrients from the arrival of LSG and their subsequent disturbance to the landscape. Collectively, findings discriminate the consequences of warming and LSG disturbance on tundra ponds from which we identify a suite of sensitive limnological and paleolimnological measures that can be utilized to inform aquatic ecosystem monitoring.

Avian-Driven Modification of Seasonal Carbon Cycling at a Tundra Pond in the Hudson Bay Lowlands (Northern Manitoba, Canada)

Abstract The past ∼40 years have seen a geometric increase (5–7% per year) in the size of the lesser snow goose (LSG; Chen caerulescens caerulescens) population and marked spatial expansion of the area they inhabit within the coastal fen ecotype of Wapusk National Park (Hudson Bay Lowlands, northern Canada), raising concerns and uncertainty about the environmental effects of their activities (grubbing of vegetation, soil disturbance, deposition of feces) on the abundant shallow tundra ponds. In this study, we use conventional limnological measurements as well as water and carbon (C) isotope tracers to explore similarities and differences in seasonal patterns of hydrological, limnological, and biogeochemical conditions of 15 shallow coastal fen ponds that currently have minimal (if any) disturbance from the LSG population with one pond (WAP 20) that is subject to substantial LSG activity. Carbon isotope measurements reveal that C cycling at WAP 20 (LSG-disturbed site) is markedly different compared to the other ponds, whereas only small differences were observed in hydrological conditions and concentrations of major nutrients and chlorophyll a of pond water. A mid-summer decrease in C isotope composition of dissolved inorganic carbon (DIC) occurred at WAP 20, likely as a consequence of high pond-water pH and intense C demand by aquatic productivity. These conditions appear to have promoted “chemically enhanced CO2 invasion,” which causes strong kinetic C isotope fractionation. High C demand at WAP 20 is also suggested by mid-summer 13C enrichment in particulate organic matter. In contrast, the ponds with little to no LSG activity exhibited expected seasonal C isotope behavior (i.e., 13C enrichment of DIC) under conditions of increasing productivity when C is in relatively low demand. Small differences in nutrient concentrations may be due to rapid uptake by the benthic mat at WAP 20. Data from the low disturbance ponds also provide baseline information for future studies assessing potential effects of LSG.

Surface and subsurface phosphorus export from agricultural fields during peak flow events over the nongrowing season in regions with cool, temperate climates

In cool temperate regions with significant winter periods, midwinter thaws and the spring freshet are critical periods for annual phosphorus (P) loss from agricultural fields. The efficacy of best management practices during the winter period is not clear. This paper reports hydrologic and biogeochemical P losses in surface runoff and tile drainage from two fields in Ontario, Canada, during peak flow events (~5 per site) occurring throughout the nongrowing season (October to April). We relate inter-event variability in the quantity and speciation (dissolved or particulate) of P to event climatic drivers (e.g., rainfall, rain-on-snow, and snowmelt) and pre-event soil conditions (e.g., presence or absence of snow cover or presence of frozen ground). Runoff and P (dissolved and particulate) were lost via both surface and subsurface (tile) pathways; however, P concentrations were greater and more variable in surface runoff than in tile drain effluent. The total P load leaving the sites with both overland runoff and tile drainage during the October through April period observed ranged from 0.23 to 0.34 kg ha−1 (0.21 to 0.30 lb ac−1). Particulate P concentrations, particularly in surface runoff, increased as the proportion of precipitation that fell as rainfall increased, and flow-weighted mean concentrations of particulate P were greatest when rain fell on thawed, bare soils. Particulate P represented between 70% and 90% of the total P loss in the observation period. In contrast, while dissolved reactive P loads were only 10% to 30% of the total P loss, dissolved reactive P concentrations were greatest in January and declined over the remainder of the season. This work provides insight for the design and implementation of suitable best management practices for the mitigation of P losses throughout the nongrowing season.

Subsurface Mobilization of Phosphorus in an Agricultural Riparian Zone in Response to Flooding from an Upstream Reservoir

Riparian wetlands can act as both phosphorus (P) sources and sinks depending upon a range of factors that affect hydrological and biogeochemical processes that govern P mobilization. Stream flow, groundwater levels and water chemistry (total P (TP), soluble reactive P (SRP)) were measured in a series of nested piezometers along three transects located in a riparian zone prior to and throughout a flood event resulting from the release of water from an upstream reservoir. Results of the study show that the stream was influent on all sampling dates, and groundwater flow through the riparian zone was longitudinal to the channel, rather than transverse to the stream. This drainage pattern affected riparian zone biogeochemistry. The riparian zone was a source of TP and SRP to the shallow groundwater system under both pre-flood and flood conditions, as P levels were higher in piezometers at the downstream end of the riparian zone (p <0.001). Flooding induced a brief increase in TP concentrations in shallow groundwater due to mixing with surface runoff following overbank flooding; however, these concentrations quickly returned to pre-event levels. In contrast, SRP concentrations in shallow groundwater decreased during flooding, likely resulting from mixing with oxygen-rich stream water. A large pulse of TP (12,000 g L1) was observed in the creek on the peak flood date. This P did not originate from the reservoir and was more likely due to the mobilization of P from the riparian zone surface when overbank flooding occurred. The results indicate that autumn flooding of riparian zones downstream from impoundments may mobilize phosphorus if overbank flooding occurs, thereby reducing the nutrient retention potential of riparian zones in some settings.

Seasonal dynamics in shallow freshwater pond‐peatland hydrochemical interactions in a subarctic permafrost environment

Terrestrial and aquatic ecological productivity are often nutrient-limited in subarctic permafrost environments. High latitude regions are experiencing significant climatic change, including rapid warming and changing precipitation patterns, which may result in changes in nutrient dynamics within terrestrial and aquatic systems and hydrochemical transport between them. The objective of this research was to characterize changes in runoff quantity and quality within, and between peatlands and ponds throughout the snow-free summer season. Two ponds and their catchments were monitored over the snow-free season to measure changes in hydrologic storage, and to determine how water chemistry changed with the evolution of the frost table depth. Thresholds in hydrologic storage combined with frost table position (which inhibited infiltration and storage) produced non-linear responses for runoff generation through highly-conductive shallow peat layers while deeper, less conductive layers retarded flow. Greater inputs were required to exceed hydrologic storage (fill and spill) as a deepening frost table increased the hydrologically active portion of the soil, leading to seasonal variability in runoff pathways between peatlands and ponds. Runoff contributions to ponds were an integral component of the snow-free water balance during the study period, contributing up to 60% of all snow-free inputs. Groundwater chemistry (and pond chemistry following runoff events when ponds were connected with peatlands) reflected the different depths of peat and mineral soil accessed throughout the season. This work has improved scientific understanding of the combined controls of hydrologic inputs and ground frost on runoff and nutrient transport between peatlands and ponds, and sheds insight into how nutrient dynamics in cold regions may evolve under a changing climate. This article is protected by copyright. All rights reserved.

Advances in Canadian Research Coupling Hydrology and Water Quality, 2003-2007

Canada is a country of considerable fresh water resources, with a mature community of hydrologists whose research is concerned with the transport and fate of nutrients and other pollutants that impact water quality, either from a human use or ecosystem perspective. This work has led to important advances in understanding that have contributed to regulatory action with sometimes global impact. However, water quality issues persist, largely due to increased natural resource extraction, land-use change, climate change and the identification of emerging contaminants of concern. This paper highlights important advances by Canadian researchers coupling hydrology and water quality over the period 2003-2007.