Introduction
Coastal systems are increasingly subject to environmental degradation due to stressors of both natural and anthropogenic origin. One stressor, nutrient loading, contributes to algal blooms and subsequent decomposition that can limit dissolved oxygen in the water column (Pinckney et al., Reference Pinckney, Paerl, Tester and Richardson2001). Macrobenthos are particularly sensitive to organic enrichment, making these communities useful for studying the influence of nutrient inputs on a system (Pearson & Rosenberg, Reference Pearson and Rosenberg1978). Oso Bay, Texas, USA, is a shallow (mean 1 m), estuarine secondary bay in the northwestern Gulf of Mexico (Figure 1) experiencing rapid urbanization coincident with increased nutrient and organic matter inputs (Wang et al., Reference Wang, Hu, Wetz and Hayes2018; Wetz et al., Reference Wetz, Hayes, Fisher, Price and Sterba-Boatwright2016). Microtidal, low-flow estuaries such as Oso Bay are believed to be more sensitive to anthropogenic influences than “classical” estuaries with strong riverine and/or tidal influences due to limited flushing (Bricker et al., Reference Bricker, Longstaff, Dennison, Jones, Boicourt, Wicks and Woerner2007). This sensitivity makes management of these systems more challenging.
Figure 1. Sampling sites within Oso Bay. WTP = Wastewater treatment plant.
Objective
The aim of this 11-month study was to analyze spatial changes in estuarine macrobenthic communities adjacent to regions that varied in land cover. Oso Bay has multiple stressors, including its primary tributary, Oso Creek, being dominated by discharge of hypersaline cooling water from a power plant as well as discharge from multiple municipal wastewater treatment plants, while another wastewater treatment plant discharges directly into Oso Bay. This study focuses on spatial differences in land cover and the influence of nutrient inputs on macrobenthic communities to inform ongoing management activities by state (Texas Commission on Environmental Quality) and federal (U.S. Environmental Protection Agency) environmental management agencies.
Methods
Macrobenthic infaunal communities and sediment chlorophyll concentrations were collected using core samples monthly from February 2013 to January 2014 (de Santiago et al., Reference de Santiago, Palmer and Beseres Pollack2020). Sediment grain size was sampled in January 2014. Water quality measurements (salinity, temperature, dissolved oxygen, pH) and water chemistry samples (SiO42−, NH4+; NO2 + NO3, PO43−, dissolved organic carbon [DOC], dissolved organic nitrogen [DON], total dissolved nitrogen [TDN]) were taken three to five days prior to benthic sampling (Wetz et al., Reference Wetz, Hayes, Fisher, Price and Sterba-Boatwright2016).
Sampling occurred at six sites within Oso Bay, including the head of Oso Bay below a hypersaline discharge point to Oso Creek (Yorktown Bridge, YB), and the mouth at Oso Bay inlet (OI), as well as four sites in the western region of the bay: an active golf course that uses reclaimed wastewater from for watering (AG), a defunct golf course (DG), a mix of agricultural land and impervious surface (AI), and a mixed residential area with outflow dominated by effluent from a municipal wastewater treatment plant (WP) (Figure 1). Multivariate statistical techniques (non-metric multidimensional scaling (nMDS), principal component analysis (PCA), Bio-Env; Clarke & Ainsworth, Reference Clarke and Ainsworth1993) and Pearson’s correlations were used to characterize macrobenthic community composition and water quality.
Results
Macrobenthic communities are clustered into two groups: the four sites in the western region of the bay, and the stations at the inlet and head of the bay (Figure 2, Figure S1). The four sites in the western region were characterized by having high abundances of ostracods and oligochaetes but low abundances of bivalves and the polychaetes Mediomastus californiensis, Brania sp. and Scoloplos sp. (Suppl. Table 1). Mean ostracod abundance at WP was ≥50 times greater than at any other site.
Figure 2. Non-metric multidimensional scaling plot of mean community composition at each station.
Nutrient concentrations decreased and salinity increased with distance away from WP (Figure 3). Spatial variations in macrofaunal community structure are most highly correlated with silicate concentrations (r = 0.428, p = 0.001), followed by the combination of silicate, DOC and salinity (r = 0.407, p = 0.001; Bio-Env). Mean silicate concentrations were 2-11X higher at WP, AG and DG than AI, OI and YB (Figure S2, Table S2). Silicate is negatively correlated with salinity and dissolved oxygen and positively correlated with nutrients (NH4+; NO2 + NO3, PO43−, DOC, DON, TDN; Table S3).
Figure 3. Principal components analysis of spatial variation in water quality and sediment chlorophyll. Variable vectors (top) and loading scores (bottom).
Discussion
Oso Bay has experienced rapid urbanization in recent decades coincident with shifts in water quality and symptoms of eutrophication (Bugica et al., Reference Bugica, Sterba-Boatwright and Wetz2020). Macrobenthos community composition corroborates with previous water quality results indicating impairment of the western region of the bay due to the effluent-based nutrient inputs at WP (Wang et al., Reference Wang, Hu, Wetz and Hayes2018; Wetz et al., Reference Wetz, Hayes, Fisher, Price and Sterba-Boatwright2016). The macrobenthic community in the nutrient-rich western region of the bay was dominated by ostracods, known indicators of reduced water quality and sewage discharge (Rosenfeld & Ortal, Reference Rosenfeld, Ortal and Maddocks1983), and oligochaetes, commonly used as indicators of organic pollution (McLusky et al., Reference McLusky, Teare and Phizacklea1980). Wastewater discharge can also be a significant source of silicate (Clark et al., Reference Clark, Simpson, Bopp and Deck1992; Van Dokkum et al., Reference Van Dokkum, Hulskotte, Kramer and Wilmot2004), which was highest in the western region of the bay and most strongly influenced macrobenthic community composition over space and time. Although the OI and YB sites are most separated in space, upstream hypersaline discharge appears to facilitate a macrobenthic community at YB that is similar to OI.
Conclusion
The dominant point source of nutrients and fresh water in the western region of Oso Bay (wastewater treatment plant) influences macrofaunal communities. Despite nutrient concentrations decreasing away from WP, macrobenthic communities remain more similar than those occurring at the head and inlet of the bay, likely due to relatively lower rates of water exchange in the western region. This correlation between macrofauna and water quality in Oso Bay indicates that macrobenthic communities are influenced by nutrient loading in shallow, microtidal estuaries, and can successfully be used as ecological indicators. The data that support the findings of this study are openly available in GRIIDC (de Santiago et al., Reference de Santiago, Palmer and Beseres Pollack2020).
Acknowledgements
The authors thank two anonymous reviewers for constructive comments that improved the quality of this manuscript. We also thank Ken Hayes, Kelsey Fisher, Lynn Price and several others for collecting and analyzing water quality samples and Alex Nunez for management insights. We are grateful to Diana Sokoly for assistance with Oso Bay field sampling. Lastly, we thank the Gulf of Mexico Research Initiative Information and Data Cooperative for storing, and allowing open access of, the data that were generated in this study (de Santiago et al., Reference de Santiago, Palmer and Beseres Pollack2020).
Author Contributions
JBP, KDS, and MSW conceived and designed the study. KDS and MSW conducted data collection. KDS and TAP performed statistical analyses. TAP and JBP wrote the article.
Funding Information
This project was funded in part by a grant approved by the Texas Land Commissioner pursuant to National Ocean and Atmospheric Administration (NOAA) award no. NA21NOS4190021, and by Institutional Grant no. NA14OAR4170102 to the Texas Sea Grant College Program from the National Sea Grant Office, NOAA. All views, opinions, findings, conclusions, and recommendations expressed in this paper are those of the author(s) and do not necessarily reflect the opinions of the NOAA, Texas Sea Grant College Program or any subagencies of NOAA. Additional funding was provided by the Coastal Bend Bays & Estuaries Program.
Conflict of Interest
KDS, TAP, MSW and JBP declare none.
Supplementary Materials
To view supplementary material for this article, please visit http://dx.doi.org/10.1017/exp.2020.44.
Open access
Comments
Comments to the Author: This succinct article is well written with a clear take home message. It should prove interesting to other researchers and resource managers in the field of aquatic environmental science. Some suggestions for the authors to consider:
Abstract
- A connection should be made between the land cover and the stressors of concern
- what is the mechanistic connection between silicate as a proxy and other stressors shown to be correlated with silicate?
Introduction
-The role/connection of land cover is overlooked in the Introduction
Objective
-Connection between land cover and nutrients and other stressors is not made
-The temporal aspect of ‘spatiotemporal’ not addressed explicitly
Methods
-The temporal aspect of ‘spatiotemporal’ is not addressed explicitly, its only implied in the methods
- No details at all about the statistical analysis – at least should mention what was performed with which samples and in what context
Results
- No reference to the temporal dimension while using the term, ‘spatiotemporal’, except as implied in the NMDS graph of multiple points per station – was there a temporal pattern?
-Oligochaetes are known to be indicators of organic enrichment in low salinities
-No explicit connection to land cover is made
- Are correlations in table S3 adjusted for multiple testing error? (perhaps give threshold in legend); reviewer recognizes referring to P values as ‘statistically significant’ is out of favor, but the problem of multiple testing as an influence on P values remains
Discussion
-Oligochaetes are known to be indicators of organic enrichment in low salinities
-Lack of referral to any connection with land cover
-No discussion of temporal dimension for a study that lasted one year
Conclusion
-Role and promise of silicate as an indicator?
-Land cover?
-Temporal dimension?