Monitoring nutrient-induced hypoxia and its effect on the invertebrate community in the southern Gulf of St. Lawrence, Canada

Nutrient-induced impacts (eutrophication) in estuaries and coastal systems pose one of the greatest threats to aquatic ecosystem health. Given the economic, cultural and ecosystem importance of coastal systems, factors negatively impacting them have been a subject of study for decades. However, objective methods for quantifying the cumulative effects of nutrients have been elusive and, to date, inconsistent among studies and regions. This dissertation is composed of four data chapters of novel research to address the measurement and biological impacts of dissolved oxygen using high frequency data loggers at multiple estuaries and at a regional scale. The hypothesis tested in Chapter 2 was that high frequency dissolved oxygen measurements are an effective tool for discriminating between trophic status of estuaries of the southern Gulf of St. Lawrence, Canada. This was examined by determining whether the amount of dissolved oxygen was predicted by nutrient loading and water residence time. Using six months of raw dissolved oxygen data from 15 southern Gulf of St. Lawrence estuaries, metrics were created that reflect symptoms of eutrophication and then analyzed using multivariate techniques. Ultimately, these metrics proved effective at discriminating between habitat type (algae or seagrass dominated) and were correlated with nitrate-N and water residence time. While dissolved oxygen measured in the upper estuary was successful at discriminating among estuaries it was unclear if those measurements are indicative of impacts at other spatial scales, leading to the second chapter. The primary hypothesis tested in Chapter 3 was that dissolved oxygen parameters in the upper estuary would relate to the severity of hypoxia on a larger spatial scale in the estuary. To examine the effect of spatial scale, dissolved oxygen was monitored along the salinity gradient from the upper estuary to the outer estuary, i.e. where the estuary entered a bay or the Northumberland Strait. In general, the upper estuary experienced symptoms of eutrophication earlier and sustained them longer than farther out the estuary, although impacts were observed farther out the estuary at the most impacted estuaries. Temperature and tidal amplitude were hypothesized to influence dissolved oxygen within an estuary, but only temperature had a significant effect and only in the upper estuary. The hypothesis tested in Chapter 4 was that invertebrate communities in the upper estuary would be affected by both hypoxia and the dominant vegetation there. Specifically, this study examined the influence of fine-scale dissolved oxygen on the invertebrate community in Ulva spp. and Zostera marina habitat. The invertebrate community was distinct between vegetation types and, in general, communities were more similar within than among estuaries. The exception was if the community had recently been exposed to sustained hypoxia/anoxia in the 48 hours prior to sampling. Communities affected by hypoxia were more similar to each other, regardless of site, and were differentiated from other communities by a higher proportion of hypoxia tolerant animals, e.g. some bivalves and gastropods, and a lower proportion of crustaceans. During field work for Chapter 4 it was observed that amphipods tended to swim throughout the water column during periods of hypoxia/anoxia and it was hypothesized that they may be using surface floating algal mats as refugia. Thus, the hypothesis tested in Chapter 5 was that invertebrate communities would differ between floating and submerged mats of Ulva spp.. Ultimately, floating mat communities were found to be distinct from submerged mats, mostly due to a greater occurrence of rare species. An increase in the proportion of epibenthic invertebrates on floating mats, relative to submerged mats, immediately following hypoxia supported the initial hypothesis. The primary implication was that some epibenthic invertebrates are capable of escaping hypoxia/anoxia through rafting and that they can potentially repopulate impacted areas through the same mechanism. The overall objective of this dissertation was to develop scientifically sound methods for monitoring eutrophication. Ultimately, hourly measurements of dissolved oxygen in the upper estuary proved more sensitive, and less labour intensive, compared to monitoring macroinvertebrate communities and to many other methodologies used to assess eutrophication found in the literature. Refining nutrient loading, external and internal, and water residence time models used in this study should improve the relationship between predictor variables and dissolved oxygen in the upper estuary and at other spatial scales