Beneath the surface: spatial and temporal trends in water quality and its impacts on algal community composition in the Albemarle Sound, North Carolina

Abstract

Urban and agricultural expansion and intensification pose a critical threat to water quality and aquatic ecosystems. Increased nutrient loading into waterways combined with warming temperatures due to climate change have increased eutrophication and algal blooms. The relationship between land use, nutrient availability, and algal growth can vary dramatically across space and time, but few studies have captured this variation. The goal of this research is to assess water quality across time and disparate land uses, and its influence on algal community composition in the Albemarle Sound, a brackish water estuary in North Carolina. We collected water quality data from 21 sites across the sound, visiting six sites in Chowan County biweekly and 15 other sites twice between June and August 2020. Water samples from each site were tested for nitrate, phosphate, ammonia, bicarbonate, and total phosphorus (TP). Preserved algal samples from the six Chowan County sites were enumerated under a microscope to estimate genus richness and biomass. In the Chowan County sites, phosphorus increased and nitrate decreased over the course of the summer. Across all sites, TP increased with development and agricultural land use. These results suggest that sources of nitrogen and phosphorus in the sound differ. Algal richness increased with nitrate concentration and decreased with precipitation while biomass increased with water temperature. Our results indicate that climate change impacts, particularly increasing temperatures and extreme precipitation, influence how land use, water quality, and algal community composition interact. These data demonstrate the co-benefits of mitigating climate change in developing management strategies to reduce algal blooms.

Supplementary information: The online version contains supplementary material available at 10.1007/s10452-023-10008-y.

Keywords: Algae; Community composition; Land use; Nutrient concentration; Water quality.

Fig. 1

Map of study region. Sampling was conducted at 21 sites across 8 counties in Northeastern North Carolina. Sites in Chowan County (1–6) were sampled every two weeks from June 8th to August 10th for a total of five times while the remaining sites (7–21) were each sampled twice across June and July. The watersheds for each site are outlined in green. We obtained state outlines from the USA Census Bureau (2018) and county outlines from the NCDEQ (2019)

Fig. 2

NO₃⁻ (left) and TP (right) concentrations and average weekly precipitation. The figures illustrate the NO₃⁻ and TP concentrations at each site and precipitation over the summer. Bars represent average weekly rainfall, patterned lines indicate different sites (n = 6), and points represent visits to each site (n = 30; n = 24). During the last week, Hurricane Isaias hit the region, causing a dramatic rise in precipitation from the previous week

Fig. 3

Relationship between proportion of agricultural land and TP. Points represent study sites (n = 21), best fit line shows the relationship between TP and proportion farmland, and shading indicates a 95% confidence interval. There was a significant, positive association between TP and proportion agricultural land cover in site watersheds among all study sites (p < 0.05). See Table 3 for slope based on full model

Fig. 4

Relationship between algal genus richness and time by study site. Points represent visits to each site (n = 26); color-coded best fit lines represent different sites (n = 6). The linear modeling results indicate that algal genus richness increased significantly over time (p < 0.001), though site was not included in the best model

Fig. 5

Relationship between total biomass (as biovolume) and water temperature. Points represent visits to each site (n = 26); best fit line represents the relationship between algal biomass and water temperature, and gray shading represents a 95% confidence interval. Algal biomass (measured as biovolume) increased significantly with water temperature (p < 0.001). See Table 5 for slope based on full model

Fig. 6

Proportion biomass of algal groups including unknowns over time at each site. Stacked bar plots show the proportion biomass (measured as biovolume) of the 5 algal groups identified in this study and unknowns. The first two algal samples at sites 5 and 6 (collected on June 13th) were omitted from this chart as algal taxa data was not collected for the remaining four sites until the following week

Fig. 7

Relationship between nitrate concentration and relative cyanobacteria (left) and dinoflagellate (right) biomass (measured as biovolume). Points represent visits to each site (n = 26); best fit lines represent the relationships between nitrate concentration and cyanobacteria and dinoflagellate relative biomass, and gray shading represents a 95% confidence interval. Relative cyanobacteria biomass increased significantly with NO₃⁻ concentration (p < 0.01) while percent biomass of dinoflagellates decreased (p < 0.05). See Table 6 for slopes based on full model