Influence of nutrient enrichment on temporal and spatial dynamics of dissolved oxygen within northern temperate estuaries

Abstract

In temperate estuaries of the southern Gulf of St. Lawrence, intermittent seasonal anoxia coupled with phytoplankton blooms is a regular occurrence in watersheds dominated by agricultural land use. To examine the spatial relationship between dissolved oxygen and phytoplankton throughout the estuary to assist in designing monitoring programs, oxygen depth profiles and chlorophyll measurements were taken bi-weekly from May to December in 18 estuaries. In five of those estuaries, dissolved oxygen data loggers were set to measure oxygen at hourly intervals and at multiple locations within the estuary the subsequent year. The primary hypothesis was that dissolved oxygen in the upper estuary (first 10% of estuary area) is predictive of dissolved oxygen mid-estuary (50% of estuary area). The second hypothesis was that hypoxia/superoxia in the estuary is influenced by temperature and tidal flushing. Oxygen depth profiles conducted in the first year of study provided preliminary support that dissolved oxygen in the upper estuary was related to dissolved oxygen throughout the estuary. However, dissolved oxygen from loggers deployed at 10% and 50% of estuary area did not show as strong a correlation as expected (less than half the variance explained). The strength of the correlation declined towards the end of summer. Spatial decoupling of oxygen within the estuary suggested influence of local conditions. Chlorophyll concentration seemed also to be dependent on local conditions as it appeared to be coupled with the presence of sustained anoxia in the upper estuary with blooms typically occurring within 7 to 14 days of anoxia. The practical implication for oxygen monitoring is that one location within the most severely impacted part of the estuary is not sufficient to fully evaluate the severity of eutrophication effects.

Spatial and Temporal Variation of C, N, and S Stable Isotopes and Seagrass Coverage Related to Eutrophication Stress in Zostera marina

“…Comparing their assessment to the results found herein would indicate that station 1 (where eelgrass δ 13 C was approximately −9 to −10) seemed to have a higher N load than stations 2-5 (where eelgrass δ 13 C was approximately −7 to −8). This conclusion would also be consistent with data previously presented [22,27] that found that local conditions in inner Wheatley River stations (in this case station 1) were generally poorer in quality compared with local conditions in outer stations (in this case stations 2-5). Lower light, as supported herein by increased turbidity, will lead to lower photosynthetic productivity, which is evidenced by the very low above-and below-ground biomass.…”

“…The responses of biota to this large fluctuation have implications for monitoring frequency and interpretation. According to a multi-estuary model of nitrogen loading vs. eelgrass coverage for the region [4,27], the predicted coverage based on the nutrient loading observed herein was 20.9 and 44.5% of available habitat occupied for 2014 and 2020, respectively. The predicted values differed by less than 10% of the observed values, strongly suggesting a nutrient-mediated increase.…”

Comparing Physical Collection and Environmental DNA Methods for Determining Abundance Patterns of Gammarus Species along an Estuarine Gradient

“…The uppermost sampling zones in each estuary (Upper) occurred where the estuary funnels transitioned into river channels and were mesohaline at high tide (Figure 1). Mid-estuary sites occurred at sites with salinities ~20 PSU and were located approximately at the boundary line delimiting the inland locations of 10% of the estuary areas, as previously defined in these estuaries by Coffin et al [1,13]. Lower estuary sites occurred at approximately 50% of the estuary areas, as in Coffin et al [13], and were characterized by salinities of 25 PSU (Figure 1).…”

“…Longitudinal patterns are less clear for Gammarus species in northwest Atlantic estuaries [1,11,12]. Understanding species-specific zonation is vital in quantifying anthropogenic stress, with particular concerns for inputs in upper estuaries where the salinity transition area is short and land-based stressor levels may be high [13].…”

Water Quality Index as a tool for assessing the water quality of the Laguna de Bay from 2020 to 2021

“…Nutrients also exert some degree of influence on the DO (Coffin et al 2021;Lin et al 2020) and BOD (Abu Shmeis 2018). BOD indicates the amount of oxygen needed by microbes in breaking down organic matter; high concentrations of nutrients facilitate microbial growth and hence, high demand for oxygen.…”