Agricultural land use impacts on cool-spring flora and fauna, with an emphasis on freshwater invertebrate diversity and phenology in spring pools of eastern Prince Edward Island (Canada)

Abstract

Freshwater springs are focused discharge points from groundwater to surface water environments. Cool springs have consistent temperatures close to the mean annual temperature for the region and chemical composition that can vary with land use and local geology. Animal taxa inhabiting these springs must be able to tolerate nearly constant cool temperatures (<10oC), so springs usually have lower numbers of species than reported in nearby surface waters. Agricultural activities adjacent to springs add nutrients to groundwater, and alter benthic sediment structure and adjacent riparian areas, all factors that affect populations of freshwater plants and invertebrates. High nutrients should increase invertebrate abundance, but habitat alterations such as sediment addition may depress abundance. Increased food availability can also affect growth and life history patterns such as insect emergence timing, which can be disrupted in cool springs due to lack of temperature cues to synchronize development. Agricultural impacts on springs were examined by comparing water quality and invertebrate community structure in rheo-limnocrene springs in forested and agricultural areas in Prince Edward Island, Canada. Twenty springs (10 surrounded by agricultural land and 10 forested) were monitored for water quality and nine of these (five agricultural and 4 forested) were further examined to explore invertebrate and aquatic plant patterns. Agricultural sites had open canopies, high nitrogen and sulphur levels, high amounts of fine sediment, and plant cover dominated by vascular plants. Forested sites had closed canopies, low nutrient levels, clean gravel substrates, and plant cover dominated by bryophytes. Invertebrate diversity and abundance were highest in forested springs and community structure differed between land-use types. Midges (Chironomidae) dominated the macroinvertebrate community in all sites, but several midge and mite (Hydrachnidiae) genera were most abundant in forested sites. Mayflies (Ephemeroptera) were very rare in the springs, but stoneflies (Plecoptera) and caddisflies (Trichoptera) were most abundant and diverse in agricultural sites. Emergence timing was compared between agricultural and forested sites for the stoneflies, and although most showed the asynchronous emergence periods expected for constant temperature sites, at least two species began to emerge earlier in agricultural sites than forested ones. Reduction of the riparian canopy leading to increased light levels from open cover was a better predictor of plant and invertebrate species assemblages than either nutrients or sediment patterns in agriculturally impacted springs; the higher light levels increased the presence and cover of vascular vegetation which altered the overall invertebrate community