Interrelationship between nutrients and chlorophyll-a in an urban stormwater lake

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A B S T R A C T Urban stormwater lakes in cold regions are ice-covered for substantial parts of the winter. It has long been considered that the ice-covered period is the “dormant season,” during which ecological processes are inactive. However, little is known about this period due to the historical focus on the open-water season. Recent pioneering research on ice-covered natural lakes has suggested that some critical ecological processes play out on the ice. The objective of this study was to investigate the active processes in ice-covered stormwater lakes. Data collected during a two-year field measurement program at a stormwater lake located in Edmonton, Alberta, Canada were analyzed. The lake was covered by ice from November to mid-April of the following year. The mean value of chlorophyll-a during the ice-covered period was 22.09% of the mean value for the open-water season, suggesting that primary productivity under ice can be important. Nitrogen and phosphorus were remarkably higher during the ice-covered period, while dissolved organic carbon showed little seasonal variation. Under ice-covered conditions, the total phosphorus was the major nutrient controlling the ratio of total nitrogen to total phosphorus, and a significant positive correlation existed between total phosphorus and chlorophyll-a when the ratio was smaller than 10. The results provide preliminary evidence of the critical nutrient processes in the Stormwater Lake during the ice-covered period.

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Interrelationship between nutrients and chlorophyll-a in an urban stormwater lake during the ice-covered period *Kejian Chu1, Yuntong She2, Jeff Kemp3, Mark Loewen4, Evan Davies5 1College of Environment, Hohai University, Nanjing, P. R. China 1, 2, 3, 4, 5Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Canada 1E mail: kejian@ualberta.ca , 2 E mail: yuntong.she@ualberta.ca A B S T R A C T Urban stormwater lakes in cold regions are ice-covered for substantial parts of the winter. It has long been considered that the ice-covered period is the “dormant season,” during which ecological processes are inactive. However, little is known about this period due to the historical focus on the open-water season. Recent pioneering research on ice-covered natural lakes has suggested that some critical ecological processes play out on the ice. The objective of this study was to investigate the active processes in ice-covered stormwater lakes. Data collected during a two-year field measurement program at a stormwater lake located in Edmonton, Alberta, Canada were analyzed. The lake was covered by ice from November to mid-April of the following year. The mean value of chlorophyll-a during the ice-covered period was 22.09% of the mean value for the open-water season, suggesting that primary productivity under ice can be important. Nitrogen and phosphorus were remarkably higher during the ice-covered period, while dissolved organic carbon showed little seasonal variation. Under ice-covered conditions, the total phosphorus was the major nutrient controlling the ratio of total nitrogen to total phosphorus, and a significant positive correlation existed between total phosphorus and chlorophyll-a when the ratio was smaller than 10. The results provide preliminary evidence of the critical nutrient processes in the Stormwater Lake during the ice-covered period. CONTEMPORARY URBAN AFFAIRS (2017) 1(3), 24-30. https://doi.org/10.25034/ijcua.2018.3675 www.ijcua.com Copyright © 2017 Contemporary Urban Affairs. All rights reserved. 1. Introduction Stormwater lakes support urban runoff management and prevent flooding and downstream erosion in urban areas. In cold regions, these lakes are ice-covered for substantial part or the entire winter. It has long been considered that the ice-covered period is the “dormant season” for lakes (Hampton et al., 2015), during which ecosystems subjected to low temperatures are “on hold” and most ecological processes are inactive until the environmental conditions become more conducive to the growth of aquatic organisms (Bertilsson et al., 2013). In the original Plankton Ecology Group’s (PEG) model (an influential freshwater ecological model), the ice-covered period is described as a physical suppressor of the ecosystem and essentially a “reset” button for renewal in the following spring (Sommer et al., 1986). From this perspective, most historical research had focused on the open-water period as the “growing” season, and few studies have included winter research on ice-covered lakes; thus, little is knownabout the physics, geochemistry, and biology under ice in these systems (Hampton et al., 2017). More recently, some pioneering winter lake research has shown increasing evidence that some critical ecological processes are playing out under the ice (Salonen et al. 2009, Bertilsson et al., 2013). For example in Lake Erie, the under-ice phytoplankton growth and loss rates in mid-winter were found to be as high as those of the summer months (Twiss et al., 2014). Lenard and Wojciechowska (2013) compared the phytoplankton community composition of two lakes in two consecutive winters. Both lakes favored the development of nanoplankton when they were ice covered in one winter, but produced microplankton when they were completely ice-free in the second winter. Phytoplankton community structure was found to be strongly correlated with ice thickness (Ozkundakci et al., 2016). High species diversity has been found under ice despite unfavorable conditions, including limited light availability, low water temperatures, restricted air-water gas exchange and prevention of wind-induced mixing (Salonen et al., 2009, Schröder, 2013). The concentration of nutrients and dissolved organic carbon may help to drive the plankton dynamics (Babanazarova et al., 2013). Griffiths et al. (2017) examined the shifts in diatom assemblages from ten High Arctic lakes, lakes and concluded that ice cover is likely the principle driver of some of the most important ecological changes, resulting in increased diversity and the emergence of novel growth forms and epiphytic species. With respect to winter stormwater lakes, previous studies have mostly focused on the hydrodynamic, water quality, pollutant removal performances, and operational environmental risk (e.g.,Marsalek et al., 2000, 2003; Semadeni-Davies, 2006; Tixier et al., 2012). However,ecological processes in ice-covered stormwater lakes have not received the same level of attention as the natural lakes. The objective of this study was to investigate the active processes in an ice-covered stormwater lake. Data including concentrations of nutrients, dissolved carbon, and chlorophyll-a collected during a two-year field measurement program at a stormwater lake located in Edmonton, Alberta, Canada were analyzed. The Stormwater Lake was covered by ice from November to mid-April in the following year. The differences in concentrations of total nitrogen (TN), total phosphorus (TP), dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and chlorophyll-a (Chl-a) between ice-covered and open-water seasons were explored. The correlations between these variables were analyzed using the Pearson correlation test and their correlative behaviors in ice-covered and open-water periods were compared to reveal the pattern of nutrient processes occurring under ice in the study lake. 2. Study Lake and methodology The study Stormwater Lake is located in southwest Edmonton and has an average depth of 1.78 m and a storage volume of 39,000 m3. The bathymetry of the lake together with the inlet and outlet locations are shown in Figure 1. A total of 162 water samples were collected during a two-year field measurement program between October 2013 and October 2015. Sampling locations were at the inlet and outlet locations, as well as at the corner and center of the lake. 26 samples were collected during the ice-covered period by drilling holes at monthly intervals. The water samples were sent to the Biogeochemical Analytical Service Laboratory (BASL) at the University of Alberta for measurement of the pertinent water quality parameters, including TN, TP, DOC, DIC, and Chl-a. TN and TP were analyzed by LachatQuickChem QC8500 FIA Automated Ion Analyzer (American Water Works Association, 2004, 1999), DOC and DIC by Shimadzu TOC-5000A Total Organic Carbon Analyzer (EPA 415.1 (Modified)), and Chl-a by Shimadzu RF-1501 Spectrofluorophotometer (Welschmeyer, 1994) and Varian Cary 50 Probe UV-Visible Spectrophotometer (EPA 446.0 (Modified)). The detection limits of the BASL test results are 7 ppb for TN, 1.4 ppb for TP, 0.1 ppm for DOC, 0.2 ppm for DIC and 0.2 µg/L for Chl-a.

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