Impacts of hypoxia on estuarine macroinvertebrate assemblages across a regional nutrient gradient

Abstract

In this study, we examined the effects of dissolved oxygen, via metrics based on hourly measurements, and other environmental variables on invertebrate assemblages in estuaries spanning a gradient of nutrient loading and geography in the southern Gulf of St. Lawrence, Canada. Upper areas (15-25 practical salinity units (PSU)) of 13 estuaries that were dominated by either seagrass (Zostera marina Linnaeus, 1753) or macroalgae (Ulva spp. Linnaeus, 1753) were sampled from June to September 2013. Macroinvertebrate assemblages from Z. marina were found to be distinct from Ulva assemblages for both epifauna and infauna. Small snails dominated each vegetation type, specifically cerithids in Z. marina and hydrobids in Ulva. Although Z. marina had higher species richness, approximately 70% of species were common to both habitats. Faunal communities differed among estuaries with large, within-estuary, temporal variance only observed at Ulva sites impacted by hypoxia and particularly at sites with long water residence time. Indeed, abundances varied by several orders of magnitude in Ulva ranging from zero to thousands of macroinvertebrates. There was a strong negative correlation between hypoxic or anoxic water, 48 h prior to sampling, with relative abundances of amphipods, and a positive correlation with the relative abundances of snails. As one of the first studies to use high-frequency oxygen monitoring, this study revealed probable impacts and the transient nature of hypoxia in eutrophication.

Inorganic nitrogen has a dominant impact on estuarine eelgrass distribution in the Southern Gulf of St. Lawrence, Canada

“…Temperature values were collected half hourly from dissolved oxygen loggers in the same area from May–August (mean and range presented). Data were adapted from Coffin et al (, b ) with the exception that nitrate‐N loading at Bouctouche, Cocagne, and Kouchibouguac were updated with more recent data.…”

“…Nutrient loading, water chemistry, and substrate particle size and organic matter data were available from other studies (Coffin et al , b ) and are presented in Table . The loading estimates for the New Brunswick estuaries (Cocagne, Bouctouche, and Kouchibouguac) were revised from these studies with nitrate data that became available since those studies were published.…”

“…A substantial component of the environmental data was available from parallel studies, and the collection thereof previously described (Table ; Coffin et al , b ). As eelgrass decline in the region occurs first in the upper estuary, these data were consistently collected 10% of the distance down the estuary from its start, as measured by estuarine area.…”

“…Nutrient loading was calculated from direct measurement of stream water at the head of tide as described by Coffin et al (, ). Whole estuary residence time using a tidal prism model was generated using bathymetry and tide model data (Coffin et al and described below). Light data were collected in August in triplicate also at three locations representing 10%, 50%, and 100% of the estuary by area in each estuary.…”

“…Watersheds in the Southern Gulf often experience intense agricultural land‐use (> 50% of land area) and consequently they are typified by high levels of external nitrogen loading (Bugden et al ; Jiang et al ). Reduced dissolved oxygen, including frequent anoxic events, is observed on an annual basis in many estuaries (Coffin et al ) impacting invertebrate (Coffin et al , ) and fish communities (Schein et al ). Watersheds with intensive agriculture also experience high rates of erosion leading to high total suspended solids that also have the potential to impact the estuary (Alberto et al ; Sirabahenda et al ).…”