Land-use effects on nutrient and algae in the Middle and Lower Nottawasaga River and the Minesing Wetlands

Abstract:

We carried out a monthly (June to September inclusive) sampling program during baseflow conditions to measure physical, chemical (including primary nutrients) and biological variables at 15 stations along the main branch of the Nottawasaga River (between Alliston and the river mouth at Wasaga Beach). This program included four stations within the Minesing Wetlands, a Ramsar site and Provincially Significant Wetland, which should be filtering out nutrients and sediment as the river runs through it. We also obtained water samples from the main branch of the Nottawasaga River and six of its tributaries (first and second order streams) draining three types of agricultural practices (corn row crops, soy bean row crops, cattle pasture) to determine the taxonomic affiliation of planktonic algae and periphytic algae respectively. In addition, from June to August we continuously monitored water levels along and around the Giffin-Walton-Downey Agricultural Drain network within the eastern downstream portion of the Minesing Wetlands to assess the physical mixing of the drain water with the wetland sediments and to determine the residence time of water through this drain. This program also included a near-weekly (June to August inclusive) sampling program during mostly baseflow conditions along and around the drain to detect variation in water solutes (including primary nutrients) to assess the wetlands’ water purification capacity of agricultural runoff. Results from our monthly monitoring program conducted in the main branch of the Nottawasaga River indicated that nutrient and sediment loading is the most serious water-quality problem. Almost all monitoring stations exceeded the Provincial Water Quality Objective (PWQO) for Total P (TP; 30 µg/L), Total Ammonia N (TAN; 0.02 mg/L), and E. coli (100 CFU/100 mL), while Total N (TN; 1.06 mg/L), Total Suspended Solids (TSS; 3.6 mg/L), and Turbidity (TURB; 6.1 NTU) exceeded the National Agri-Environmental Standards Initiative (NAESI) performance standards for southern Ontario at every single monitoring station. Innisfil Creek (station 02) had significantly higher mean monthly TP, TSS, and TURB compared to all other stations. As the river flows through the Minesing Wetlands, TURB remained high through this section, while nutrient concentrations initially decreased; however, after Willow Creek enters the system, TP concentrations significantly increased, and the concentration of dissolved oxygen decreased significantly. As a consequence, water quality of the Nottawasaga River actually worsened as it exited the Minesing Wetlands. Willow Creek flows through Midhurst, an area of increasing urban development, before it reaches the Minesing Wetlands and flows into Nottawasaga River. Results from the monthly planktonic algae monitoring program indicated that most areas of the Nottawasaga River are dominated by pollution-dependent or pollution-tolerant species. Both species composition and dominant species of algae varied inconsistently throughout the sampling period and through the Nottawasaga River, but were consistently dominated by one of three diatom genera: Navicula, Nitzschia or Gomphonema. Although not all species in these genera are indicative of increased nutrients or degraded water quality, many of those present in high abundance, such as N. acicularis or G. parvulum, are able to dominate in nutrient-rich waters. Planktonic algal biomass, as well as density, increased throughout the sampling season from June to September across the Nottawasaga River. The Minesing Wetlands did not appear to have an effect on algal community structure or dominance. Results from periphytic algae monitoring indicated that agricultural practice (crop-type, livestock grazing, etc) did not have a significant effect on any of the water-quality variables, periphytic ash free dry mass (AFDM), or periphyton chlorophyll-α (CHL) content in the tributaries monitored, although it is premature to draw any conclusions due to the small sample size. It is noteworthy that site 6-LC, which drains corn row crops, consistently exceeded the Provincial Water Quality Objective (PWQO) for TAN (0.02 mg/L). Results from the physical characterization of the Minesing Wetlands’ interaction with agricultural drains indicated an extremely slow movement of water along the drain toward the Nottawasaga River (<0.5 cm/s during baseflow conditions). This slow transfer rate to the Nottawasaga River resulted in the agricultural runoff residing in the drain for a minimum of 5 days. However, there was minimal interaction of the drainage ditch water with the surrounding wetland sediments, except during higher water levels when the wetland water discharged into the drainage ditch. As the agricultural runoff water travelled the length of the Giffin Drain, there were significant decreases in TP, SRP, TAN, and Total Nitrate Nitrogen (TNN) concentrations, but to varying degrees. SRP and TNN decreased by 90% over the length of the ditch, and dilution was not a factor (as revealed through conservative tracers); however, TAN and TP concentrations were still above PWQO levels before the drainage ditch water entered the Nottawasaga River. We followed a standardized global reference system to classify the Nottawasaga River Watershed into six main types of land uses that included agriculture (row crop, pasture, hay), forest, wetland, barren (beaches, transitional areas, golf courses), urban (built-up pervious and impervious surfaces, golf courses), and water. The dominant land use is agricultural (47%) and it is not surprising that agricultural runoff is one of the major sources of nutrients and sediment to the Nottawasaga River. We identified five key problem areas in the Nottawasaga River. The Innisfil Creek confluence (station 02) was associated with significantly higher mean monthly concentrations of TP (48.2μg/L), TURB (41.4 NTU), and Total Suspended Solids (TSS; 29.1 mg/L) compared to all other stations. Within the Minesing wetlands, TP (42.45 μg/L) was significantly higher and pH (7.8) was significantly lower after Willow Creek (station 10) entered the river. This tributary flows through Midhurst, an area of increasing urban development. Marl Creek contributed relatively high concentrations of TP (37.3 μg/L) at station 12; similarly, Jack’s Lake (station 13), a lentic system within the Nottawasaga River, also had higher TP (42.1 μg/L) and lower DO (6.1 mg/L) compared to other stations. The warmest stretch of river occurred after Knox Road in the town of Wasaga Beach (station 14), where concentrations of Total Ammonia Nitrogen (0.08 mg/L) and E. coli (2424 CFU/100 mL) were elevated. One third of the TSS in the river is organic in nature, indicating that majority of the turbidity in the river is inorganic. Without an additional strategic sampling program, we cannot determine if the TSS is being resuspended within the river. At this point, there is no reason to believe that water in the Downey Drain is mixing with water within the Nottawasga River except during rain events. Although farmland contributes elevated levels of primary nutrients and sediments to the river, we could not find consistent differences among the three land-use practices. Water quality in the Nottawasaga River is generally impaired and needs to be rehabilitated. In particular, the anoxic conditions at Jack's Lake and in the river within the Minesing Wetlands during July could limit use by juvenile sturgeon. To determine where rehabilitation should be focused, it would be useful to quantify the relative contributions of nutrients and suspended solids from the various sub-watersheds. Since all of the data collected during this project correspond to base flow conditions, a project designed to monitor changes in water quality conditions during storm events should be carried out in the future. With respect to how the river mixes with bay water, monitoring Nottawasga Bay was outside the scope of this project. Based on aerial photos taken on July 8 with an unmanned aerial vehicle, however, the polluted water from Nottawasaga River entered the bay and flowed along the beach northward towards Tiny Township. In areas of Nottawasga Bay outside this river plume, nutrient and algal concentrations appeared to be very low.