Microrefuges from drying for invertebrates in a seasonal wetland

Abstract

Summary Seasonally intermittent freshwater environments show large temporal changes in area andenvironmental conditions (which may be harsh). We investigated whether microhabitats that retain moisture could provide a refuge during drying in a seasonal wetland.2. We investigated occupancy by invertebrates of three potential microrefuge types: surface depressions, shallow cracks and deeper fissures in the sediment of a wetland in Western Australia. Our aims were to determine whether the assemblages occupying these microrefuges differed and whether they changed as the wetland dried and reflooded.3. Ten microrefuges of each type were sampled for invertebrates, sediment and temperature during each of three hydrological phases: the damp phase (surface waterabsent but sediment moist), the dry phase (groundwater at its lowest level) and the reflooded phase (surface water present). Sediment samples taken from each microrefuge in the damp and dry phases were reflooded in the laboratory to reveal species aestivating or present as eggs, and sediment organic matter content was measured.4. Sediment organic matter content did not change between wetland phases. The invertebrate assemblage in the microrefuges showed almost complete species turnover between phases.Invertebrate composition differed between microrefuges, and temperature in the deeper fissures was on average > 10 °C lower than in surface depressions and shallow cracks.5. Microcrustaceans and gastropods survived the drier months as resting stages in the microrefuges and either emerged or hatched from eggs upon reflooding. Several species, including isopods and caddisflies, were collected only from cracks and fissures as the wetland dried. During drying, a high diversity of carnivorous species was observed in the microrefuges. Sediment microrefuges apparently underpin resistance to drought by invertebrates in SouthLake. Those species that depend on sediment fissures to survive may be threatened by declining the groundwater table in the region. Changes to wetland hydrology and other human activities that affect wetland sediment or vegetation will affect the range of microrefuges available to invertebrates to survive drying, and may thus alter wetland biodiversity.

Fringing trees may provide a refuge from prolonged drying for urban wetland invertebrates

“…abundances declined in both habitats in response to drying. Both species respond to drying by aestivating with their bivalve shell closed (Strachan et al 2014) and it appears that the higher temperatures and prolonged dry conditions during experimental drying exceeded their capacity to withstand desiccation. The greatest survival rates following desiccation occur after slow drying that gives sufficient time for animals to adjust their metabolism (Ricci and Pagani 1997).…”

“…When inundated, they provide habitat for a diverse range of waterbird, amphibian, reptile, fish and invertebrate species (Waterkeyn et al 2008). When dry, the absence of surface water means that animals must either leave the wetland (being resilient to drying, Sim et al 2013) or must rely on resistance traits to take refuge in wetland sediments (Strachan et al 2014). Dormancy may be used by some invertebrate species to survive in dry sediments, while others have desiccationresistant eggs (Boulton et al 2014;Strachan et al 2015).…”

“…Along with regional groundwater extraction for water supply, these processes are reducing inundation periods in urban wetlands (Sim et al 2013). Connectivity to groundwater may cause patches of wetland sediment to remain damp (Strachan et al 2014). Conversely, declining groundwater tables and increased air temperatures may increase sediment water loss.…”

“…Variations in these qualities are likely to provide different opportunities for species taking refuge in sediments. For example, sediments with high organic matter content shrink as they dry, creating cracks in the sediment that may allow access to groundwater as a refuge for species (Strachan et al 2014). Furthermore, many wetlands have fringing trees (FT) that shade the sediment during dry periods, as well as contributing organic matter to the sediment, potentially increasing its capacity to retain moisture (Rawls et al 2003).…”

“…Although sediment from open water at the centre of a wetland is expected to have a longer hydroperiod, it will be exposed to higher temperatures when dry (Strachan et al 2014), potentially exceeding the tolerances of some invertebrates and reducing the diversity and abundance of assemblages emerging upon inundation.…”