Spatial and Temporal Variation in the Aquatic Invertebrate Community Structure of Rock Pools Along the Penobscot River, Maine

Pools of water that form in the fissures and depressions of rock outcrops, known as rock pools, are fairly common aquatic habitats that can easily be found along the rocky banks of many of Maine’s major rivers. In general, rock pools and the aquatic invertebrates inhabiting them have received little research attention and, though ubiquitous, have never been studied in Maine. My research addressed this knowledge gap by surveying 40 rock pools at four sites along the Penobscot River in Maine. The rock pools themselves had highly variable environmental characteristics and differed across sites and over time, especially in hydroperiod. They contained surprisingly abundant and diverse communities, totaling 71 invertebrate taxa across 16 orders. The non-biting midge Dicrotendipes and the biting midge Dasyhelea were the most abundant genera. Community composition differed significantly between sites in June, largely associated with differences in pool size, hydroperiod, influence of the adjacent river, and food resources. However, over the course of the summer, communities across sites became more similar to each other, likely due to the combination of phenologically-driven life histories for some taxa and the loss of stress-intolerant taxa. I also conducted an experiment in which I artificially extended rock pool hydroperiods to determine the independent effect of hydroperiod on invertebrate community structure. I hypothesized that pools with longer hydroperiods would contain more diverse and abundant invertebrate communities and that pools with longer hydroperiods would contain more long-lived taxa, such as Odonata and Coleoptera. To test this hypothesis, I prevented ten rock pools from desiccating by adding deionized water to them and left ten rock pools to naturally dry. Hydroperiod was not a significant driver of overall invertebrate abundance or richness and was only important in determining the individual abundance of one of five taxa collected in the experiment. Pool volume, location on the rock outcrop, and water chemistry (pH and conductivity) were the significant factors determining community structure. This suggests that the effect of hydroperiod observed in my survey and in other rock pool surveys may be confounded by pool size and by environmental variables mediated by hydroperiod