Associate Professor Institute for Watershed Studies, Western Washington University Bellingham, Washington, United States
Abstract: The relationship between early successional terrestrial and aquatic ecosystems is still little known. Newly formed ponds and hummocks created by the 1980 Mount St. Helens volcanic eruption and landslide are still in their early successional stages and provide a natural experiment to better understand terrestrial-aquatic linkages. Zooplankton communities in these ponds may experience profound habitat and resource changes due to shifting forest plant communities and climate change. As the terrestrial plant community shifts from deciduous-dominant forest to coniferous-dominated and climate change warms the region, an increase in terrestrial organic carbon, i.e. brownification, is expected to alter pond habitats.
Eighteen mesocosms were established in 300-L cattle tanks near three perennial ponds at the Mount St. Helens National Volcanic Monument to observe zooplankton community and ecosystem changes. Using a factorial design, mesocosms were treated with a browning treatment of humic acid crossed with a leaf litter treatment, consisting of litter collected from nearby conifers (Douglas fir, Pseudotsuga menziesii, and noble fir, Abies procera), or deciduous trees (red alder, Alnus rubra, and willow, Salix salicaceae). Ponds and mesocosms were sampled weekly for five weeks over the summer to examine zooplankton diversity, biomass, and ecosystem respiration. Temperature, dissolved oxygen, pH, and nutrients (phosphorus, nitrogen, carbon) were measured or collected in ponds and mesocosms weekly. Dissolved oxygen and temperature loggers were placed in mesocosms to monitor these variables continuously.
Dissolved oxygen exhibited a sharper decline in browning treatment mesocosms (average mean control = 8.28 mg/L, browning 6.42 mg/L), with the lowest dissolved oxygen levels in the browning treatments that were crossed with deciduous leaf litter (mean 2.96 mg/L). Total organic carbon in mesocosms after three weeks averaged twice as high in deciduous and browning-treated mesocosms compared to deciduous treatments alone and six-fold higher than the untreated controls, with coniferous mesocosms slightly lower than deciduous. There were differences in zooplankton community composition and biomass between the mesocosm treatments. Uncovering the terrestrial influences of plant succession and the effects of climate change in ponds can provide insight into zooplankton community structure, and ultimately, ecosystem function in this novel landscape.