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Coastal residential canals harbor distinct water quality conditions and phytoplankton community composition
(Estuarine, Coastal and Shelf Science, 2023-12-12) Jordana Cutajar; Blair Sterba-Boatwright; Michael Wetz
As urbanization increases, many regions around the globe are seeing an increase in the number of residential canal systems along their coastlines. These canals possess unique attributes that may facilitate water quality degradation, namely shallow depths, susceptibility to urban runoff, and limited flushing. Despite this, there has been little research on the water quality and phytoplankton dynamics of these systems. In this study, water quality and phytoplankton biomass/composition were quantified once per month in the fall and winter, and twice per month in the spring and summer, over a 1-year period at three sites along a mouth-interior gradient of a canal system on North Padre Island (Corpus Christi, Texas, USA). It was hypothesized that interior canal sites would exhibit symptoms of water quality degradation more regularly than a site at the mouth, and interior canal sites would also have higher chlorophyll a and distinct phytoplankton community composition compared to the mouth. Results showed that the mouth site exhibited lower inorganic nutrient concentrations and less pronounced variability in salinity and nutrients than the interior canal sites. Chlorophyll a was 2.5-3-fold higher in the canal sites than at the mouth on average, and diatoms dominated at the mouth while a more diverse assemblage dominated at the canal sites. Episodic to persistent hypoxia was observed at the canal sites, but not at the mouth. These findings show that artificial coastal residential canal systems have distinct and less desirous water quality conditions and phytoplankton communities, pointing to a need for additional research on the attributes of canals that foster these conditions to support effective ecosystem health management efforts.
Effect of Temporarily Opening and Closing the Marine Connection of a River Estuary
(Estuaries and Coasts, 2022-12-06) Montagna, Paul; Palmer, Terence; Pollack, Jennifer Beseres
The lower Rio Grande is a river-dominated estuary that serves as the border between Texas, USA, and Tamaulipas, Mexico. River estuaries encompass the section of the river influenced by tidal exchange with the Gulf of Mexico, but the connection with the Rio Grande is intermittent and can be temporarily open or closed. During the 4.8-year study period, the river mouth was closed 30% of the time, mostly during average or dry climatic conditions, with the temporary closing of the river mouth being linked to hydrology. When the Rio Grande estuary is closed, salinity is low (1.5 psu compared to 4.8 psu when open), nitrate plus nitrite are low (4.4 μM compared to 31.5 μM when open), and ammonium is high (9.6 μM compared to 4.3 μM when open), but chlorophyll is similar (20 μg/L compared to 21 μg/L when open). Benthic macrofaunal abundance and biomass are higher when the river mouth is closed: 16,700 individuals m−2 and 3.3 g m−2 compared to 8800 individuals m−2 and 2.4 g m−2 when the Rio Grande river mouth is open. Benthic macrofaunal community structure is divided into two groups: chironomid larvae and Oligochaeta dominated when the river mouth was closed, whereas polychaetes Mediomastus ambiseta and Streblospio benedicti dominated when the river mouth was open. The implications of these results for managing freshwater flows are that the open and closed conditions each have a characteristic benthic macrofaunal community that is strongly influenced by system hydrology.
A general pattern of trade-offs between ecosystem resistance and resilience to tropical cyclones
(Science Advances, 2022-03-02) Patrick, Christopher; Wetz, Michael; 43 other authors
Tropical cyclones drive coastal ecosystem dynamics, and their frequency, intensity, and spatial distribution are predicted to shift with climate change. Patterns of resistance and resilience were synthesized for 4138 ecosystem time series from n = 26 storms occurring between 1985 and 2018 in the Northern Hemisphere to predict how coastal ecosystems will respond to future disturbance regimes. Data were grouped by ecosystems (fresh water, salt water, terrestrial, and wetland) and response categories (biogeochemistry, hydrography, mobile biota, sedentary fauna, and vascular plants). We observed a repeated pattern of trade-offs between resistance and resilience across analyses. These patterns are likely the outcomes of evolutionary adaptation, they conform to disturbance theories, and they indicate that consistent rules may govern ecosystem susceptibility to tropical cyclones.
Subtropical estuarine carbon budget under various hydrologic extremes and implications on the lateral carbon exchange from tidal wetlands
(Water Research, 2022-06-15) Yao, Hongming; Montagna, Paul; Wetz, Michael; Staryk, Cory; Hu, Xinping
As coastal areas become more vulnerable to climatic impacts, the need for understanding estuarine carbon budgets with sufficient spatiotemporal resolution arises. Under various hydrologic extremes ranging from drought to hurricane-induced flooding, a mass balance model was constructed for carbon fluxes and their variabilities in four estuaries along the northwestern Gulf of Mexico (nwGOM) coast over a four-year period (2014–2018). Loading of total organic carbon (TOC) and dissolved inorganic carbon (DIC) to estuaries included riverine discharge and lateral exchange from tidal wetlands. The lateral exchanges of TOC and DIC reached 4.5 ± 5.7 and 8.9 ± 1.4 mol·C·m−2·yr−1, accounting for 86.5% and 62.7% of total TOC and DIC inputs into these estuaries, respectively. A relatively high regional CO2 efflux (4.0 ± 0.7 mol·C·m−2·yr−1) was found, which was two times the average value in North American coastal estuaries reported in the literature. Oceanic export was the major pathway for losses of TOC (5.6 ± 1.7 mol·C·m−2·yr−1, 81.2% of total) and DIC (9.9 ± 2.9 mol·C·m−2·yr−1, 69.7% of total). The carbon budget exhibited high variability in response to hydrologic changes. For example, storm or hurricane induced flooding elevated CO2 efflux by 2–10 times in short periods of time. Flood following a drought also increased lateral TOC exchange (from -3.5 ± 4.7 to 67.8 ± 17.6 mmol·C·m−2·d−1) but decreased lateral DIC exchange (from 28.9 ± 3.5 to -7.1 ± 7.6 mmol·C·m−2·d−1). The large variability of carbon budgets highlights the importance of high-resolution spatiotemporal coverage under different hydrologic conditions, and the importance of carbon contribution from tidal wetlands to coastal carbon cycling.
Phytoplankton biomass and community composition in three Texas estuaries differing in freshwater inflow regime
(Estuarine, Coastal and Shelf Science, 2022-10-31) Chin, Tiffany; Beecraft, Laura; Wetz, Michael
Because many estuaries worldwide are experiencing large-scale alterations in freshwater inflows due to climatic and human influences on watersheds, it is critical to understand ecosystem-level responses to freshwater inflow conditions and variability. This study compared environmental conditions and phytoplankton biomass/community composition among three Texas estuaries with differing freshwater inflow regimes to understand the impacts of freshwater inflow magnitude on phytoplankton communities. It was hypothesized that: 1) nutrient concentrations and phytoplankton biomass would be highest in San Antonio Bay (SA), the high inflow estuary and lower in Nueces-Corpus Christi Bay (NC) and Baffin Bay (BB) due to lower average inflows, and 2) the phytoplankton community would be dominated by large and/or fast-growing taxa in SA, with a greater fraction of small and/or slow-growing taxa in NC and BB. Highest inorganic nutrient concentrations were generally observed in SA, while high organic nutrient concentrations were found in BB. Chlorophyll a was relatively high in both SA and BB (mean 16.9–18.5 μg L−1) while phytoplankton biovolume was highest in BB. Despite distinct freshwater inflow, salinity and nutrient regimes, differences in phytoplankton community composition were less pronounced. Nano- or microplankton were the dominant size class of phytoplankton in each system, and diatoms were the dominant functional group, accounting for 27–49% of total biovolume on average. There were indications that the phytoplankton community was more diverse in SA, especially following inflow events, providing evidence that inflow may act as a disturbance that leads to greater phytoplankton diversity. Results from this study also showed that while freshwater inflow is important for nutrient delivery, low inflow estuaries such as BB are still susceptible to effects of eutrophication due to long residence times and nutrient retention/recycling. Overall, the differing responses of each of these ecosystems to freshwater inflow highlight the importance of system-specific management plans and consistent monitoring programs in coastal estuaries.
Variability in phytoplankton biomass and community composition in Corpus Christi Bay, Texas
(Estuaries and Coasts, 2022-11-14) Tominack, Sarah; Wetz, Michael
Corpus Christi Bay is a shallow, wind-driven lagoon located on the semi-arid South Texas coast that has a rapidly urbanizing watershed. Projections indicate that this region will become warmer and drier and will support an increasing urban population over the next several decades. Here, a 27-month field study was undertaken to quantify phytoplankton biovolume, community composition, and relationships with environmental drivers. Phytoplankton biovolume varied unimodally with a peak in biovolume from spring through summer followed by a decline into fall and winter. Phytoplankton growth was related to nutrient availability during the spring and summer, while water temperature and factors affecting flushing were important during the fall and winter. Regions with more restricted circulation patterns (i.e., man-made canals) were found to support higher standing crops of phytoplankton and the occurrence of high biovolume blooms. Diatoms were dominant during the winter and spring, dinoflagellates were dominant during the summer and fall, and picophytoplankton groups were important during spring, summer, and fall. These results suggest that nutrient and physical conditions interact to determine phytoplankton biomass and community composition and contribute to our ability to project potential impacts of future increases in human populations in the watershed, decreasing precipitation due to climate change, and increasing frequency of short-lived flood events.
A general pattern of trade-offs between ecosystem resistance and resilience to tropical cyclones
(Science Advances, 2022-03-02) Patrick, Christopher J.; Kominoski, John S.; McDowell, William H.; Branoff, Benjamin; Lagomasino, David; Leon, Miguel; Hensel, Enie; Hensel, Marc J.S.; Strickland, Bradley A.; Aide, T. Mitchell; Armitage, Anna; Campos-Cerqueira, Marconi; Congdon, Victoria M.; Crowl, Todd A.; Devlin, Donna J.; Douglas, Sarah; Erisman, Brad E.; Feagin, Rusty A.; Geist, Simon J.; Hall, Nathan S.; Hardison, Amber K.; Heithaus, Michael R.; Hogan, J. Aaron; Hogan, J. Derek; Kinard, Sean; Kiszka, Jeremy J.; Lin, Teng-Chiu; Lu, Kaijun; Madden, Christopher J.; Montagna, Paul A.; O'Connell, Christine S.; Proffitt, C. Edward; Reese, Brandi Kiel; Reustle, Joseph W.; Robinson, Kelly L.; Rush, Scott A.; Santos, Rolando O.; Schnetzer, Astrid; Smee, Delbert L.; Smith, Rachel S.; Starr, Gregory; Stauffer, Beth A.; Walker, Lily M.; Weaver, Carolyn A.; Wetz, Michael S.; Whitman, Elizabeth R.; Wilson, Sara S.; Xue, Jianhong; Zou, Xiaoming
Tropical cyclones drive coastal ecosystem dynamics, and their frequency, intensity, and spatial distribution are predicted to shift with climate change. Patterns of resistance and resilience were synthesized for 4138 ecosystem time series from n = 26 storms occurring between 1985 and 2018 in the Northern Hemisphere to predict how coastal ecosystems will respond to future disturbance regimes. Data were grouped by ecosystems (fresh water, salt water, terrestrial, and wetland) and response categories (biogeochemistry, hydrography, mobile biota, sedentary fauna, and vascular plants). We observed a repeated pattern of trade-offs between resistance and resilience across analyses. These patterns are likely the outcomes of evolutionary adaptation, they conform to disturbance theories, and they indicate that consistent rules may govern ecosystem susceptibility to tropical cyclones.
Impact of the Eastern oyster Crassostrea virginica on microbial community structure in a salt marsh estuary
(Inter-Research, 2002-05-16) Wetz, Michael S.; Lewitus, Alan J.; Koepfler, Eric T.; Hayes, Kenneth C.
In the tidal creeks of North Inlet, a high salinity salt marsh estuary near Georgetown, South Carolina, USA, the Eastern oyster Crassostrea virginica is an abundant component of the benthic macrofauna that exerts controls on microbial communities by its grazing and nutrient regenerative activities. The effects of oyster activity on North Inlet microbial food web structure were studied using: (1) water samples collected from tidal creeks with oyster reefs versus tidal creeks without oyster reefs (removed as part of a large-scale field manipulation study); and (2) flow-through flumes. In pair-wise comparisons of creeks with similar hydrography and morphology, the only microbial group found to vary significantly with the presence of oyster reefs was the phototrophic nanoflagellates (pflags), which were 1.25- to 2.25-fold less abundant in creeks with oyster reefs during the summer phytoplankton bloom. Because heterotrophic nanoflagellates (hflags) did not vary in these same comparisons, we hypothesized that preferential feeding for pflags by oysters was responsible for the reduction in pflag abundance. The hypothesis was tested during March and July 1999 using flumes with flowing creek water containing either live oysters or dead oyster shells. Significant reductions in pflags and some types of diatoms were measured in the outflow from live oysters, but oyster effects on other microbial components (hflags, cyanobacteria, and heterotrophic bacterioplankton) were not evident. The flume study demonstrated preferential feeding by oysters on pflags using naturally occurring microbial assemblages. The differences in pflag abundance in creeks with oyster reefs versus creeks without oyster reefs suggests that this grazing activity can affect the structure of natural microbial communities.
Response of bacteria to simulated upwelling phytoplankton blooms
(inter-research, 2004-05-19) Wetz, Michael S.; Wheeler, Patricia A.
Until recently, studies of the fate of primary production in coastal upwelling systems have focused mainly on export through sinking of particulate organic matter (POM). In week-long deck incubations conducted during the upwelling season off Oregon, a large accumulation of carbon-rich (C:N ≥ 16) dissolved organic matter (DOM) occurred following nitrate depletion by diatom blooms. The response of bacterioplankton to the DOM release in the incubations was observed using flow cytometric analysis of abundances of bacteria with high nucleic acid (HNA) and low nucleic acid (LNA) content. Relatively small increases in the abundance of HNA bacteria were observed in nitrate-replete conditions (<1.0 × 106 cells ml-1). Coincident with nitrate depletion and accumulation of the DOM, abundances and growth rates of HNA bacteria increased rapidly while little response was observed from LNA bacteria. Although growth rates and abundances of HNA cells increased markedly, a net decrease in dissolved organic carbon (DOC) was observed in only 1 incubation. Within approximately 1 d of nitrate going to depletion, HNA bacterial abundances peaked and then decreased rapidly, possibly due to flagellate grazing or viral lysing. These results indicate that on the timescale of upwelling/relaxation events, which generally last 7 to 10 d, environmental controls on bacterial populations may prevent complete degradation of phytoplankton-derived DOM, thus allowing some of this material to be exported from the system through physical processes following termination of the upwelling event.
Light-induced growth of phytoplankton collected during the winter from the benthic boundary layer off Oregon, USA
(Inter-Research, 2004-10-14) Wetz, Michael S.; Wheeler, Patricia A.; Letelier, Ricardo M.
Despite the importance of the spring phytoplankton bloom off Oregon as a food source for zooplankton, little is known about the sources of phytoplankton seed stock for the bloom or its timing. Experiments were conducted in the late winter to determine if the benthic boundary layer (BBL) could be a source of viable phytoplankton and to determine to what extent their growth was limited by light. Water collected on 3 dates from the BBL over the inner-shelf and mid-shelf in January 2003 was used for growth experiments run in shipboard incubators held at 10°C. The water was exposed to in situ nutrient concentrations and to 4 light levels and 2 daylengths (9 and 12 h), representative of winter and spring daylengths. Significant increases in chl a and particulate organic matter (POM) were observed in high light and medium light treatments at both daylengths. Chl a increased minimally (≤0.3 µg l-1) in the low light treatments, while no significant increase in POM occurred. Chl a decreased in the control (no light) treatment throughout the experiments. Blooms that developed during these incubations were dominated by Thalassiosira spp., Actinopytchus sp., Asterionellopsis glacialis and other diatoms, consistent with in situ phytoplankton community composition during the spring. Growth occurred at light levels that were generally 40 to 50% of that measured in surface waters in January, suggesting that light levels at the surface are conducive to growth, but that mixing prevents significant growth during the winter.
Variability in phytoplankton pigment biomass and taxonomic composition over tidal cycles in a salt marsh estuary
(Inter-Research, 2006-08-29) Wetz, Michael S.; Hayes, Kenneth C.; Lewitus, Alan J.; Wolny, Jennifer L.; White, David L.
Tidal flow causes high temporal variability in environmental properties that impact ecosystem dynamics. Microbes such as phytoplankton are especially susceptible to tidal advection and mixing, and understanding their role in estuarine food webs and biogeochemical cycles requires information on their biomass and taxonomic composition over short time scales (e.g. tidal cycles). We conducted a survey of phytoplankton pigment biomass and taxonomic composition over complete tidal cycles in 2 salt marsh creeks on 5 sampling occasions from July to September 2000, and assessed environmental factors regulating phytoplankton properties. Tidal input of low chl a water combined with phytoplankton losses (microzooplankton grazing, oyster grazing, settling) caused large decreases in phytoplankton biomass (by 47 to 51% on average) on the flood tide, and also influenced the taxonomic composition. Depending on sampling date, pennate diatoms or flagellates were primarily reduced on the flood tide. One sampling date followed a heavy rain event, and was marked by substantial increases in tidal creek nutrient concentrations and reduced microzooplankton grazing rates, emphasizing the need to consider the combined influences of nutrients and grazing in explaining bloom formation following rain events. The high tidal variability in phytoplankton properties suggests that strict attention to tidal phase is needed in determining long-term trends or inter-estuary comparisons in phytoplankton biomass, and primary production in tidally-driven estuaries.
MERIS Retrieval of Water Quality Components in the Turbid Albemarle-Pamlico Sound Estuary, USA
(MDPI, 2011-04-01) Sokoletsky, Leonid G.; Lunetta, Ross S.; Wetz, Michael S.; Paerl, Hans W.
Two remote-sensing optical algorithms for the retrieval of the water quality components (WQCs) in the Albemarle-Pamlico Estuarine System (APES) were developed and validated for chlorophyll a (Chl). Both algorithms were semi-empirical because they incorporated some elements of optical processes in the atmosphere, water, and air/water interface. One incorporated a very simple atmospheric correction and modified quasi-single-scattering approximation (QSSA) for estimating the spectral Gordon’s parameter, and the second estimated WQCs directly from the top of atmosphere satellite radiance without atmospheric corrections. A modified version of the Global Meteorological Database for Solar Energy and Applied Meteorology (METEONORM) was used to estimate directional atmospheric transmittances. The study incorporated in situ Chl data from the Ferry-Based Monitoring (FerryMon) program collected in the Neuse River Estuary (n = 633) and Pamlico Sound (n = 362), along with Medium Resolution Imaging Spectrometer (MERIS) satellite imagery collected (2006–2009) across the APES; providing quasi-coinciding samples for Chl algorithm development and validation. Results indicated a coefficient of determination (R2) of 0.70 and mean-normalized root-mean-squares errors (NRMSE) of 52% in the Neuse River Estuary and R2 = 0.44 (NRMSE = 75 %) in the Pamlico Sound—without atmospheric corrections. The simple atmospheric correction tested provided on performance improvements. Algorithm performance demonstrated the potential for supporting long-term operational WQCs satellite monitoring in the APES.
Environmental controls upon picophytoplankton growth and biomass in a eutrophic estuary
(Inter-Research, 2011-03-31) Wetz, Michael S.; Paerl, Hans W.; Taylor, Christopher; Leonard, Jeremy A.
We assessed the role of top-down versus bottom-up factors in regulating picophytoplankton (PicoP) growth and overall phytoplankton size structure in a eutrophic estuary. PicoP biomass reached an annual maximum in summer/fall and was positively correlated with temperature. Ephemeral blooms (chlorophyll a > 20 µg l–1) of PicoP were observed in the upper and middle regions of the estuary despite inorganic nitrogen concentrations <1 µmol l–1. Nutrient-amended PicoP growth rates were similar to in situ growth rates in the upper estuary, and PicoP biomass was negatively correlated with river-derived inorganic nitrogen concentrations, indicating that regenerated nutrients are a major source of nitrogen supporting PicoP growth. Microzooplankton grazing rates routinely exceeded PicoP growth rates during summer; therefore, grazing must have become uncoupled from PicoP growth on timescales shorter than the interval between grazing experiments (i.e. 2 to 4 wk) for PicoP to have bloomed. Field data point to the possibility of trophic cascades involving copepods, protistan grazers, and phytoplankton as a mechanism for this growth–grazing uncoupling. These and other recent findings indicate that bottom-up factors alone cannot explain the PicoP blooms observed in some estuarine systems and emphasize the need for grazing control studies to better understand the regulation of primary production.
Effects of Nitrogen Availability and Form on Phytoplankton Growth in a Eutrophied Estuary (Neuse River Estuary, NC, USA)
(PLoS ONE, 2016-08-09) Cira, Emily K.; Paerl, Hans W.; Wetz, Michael S.
Nitrogen availability and form are important controls on estuarine phytoplankton growth. This study experimentally determined the influence of urea and nitrate additions on phytoplankton growth throughout the growing season (March 2012, June 2011, August 2011) in a temperate, eutrophied estuary (Neuse River Estuary, North Carolina, USA). Photopigments (chlorophyll a and diagnostic photopigments: peridinin, fucoxanthin, alloxanthin, zeaxanthin, chlorophyll b) and microscopy-based cell counts were used as indicators of phytoplankton growth. In March, the phytoplankton community was dominated by Gyrodinium instriatum and only fucoxanthin-based growth rates were stimulated by nitrogen addition. The limited response to nitrogen suggests other factors may control phytoplankton growth and community composition in early spring. In June, inorganic nitrogen concentrations were low and stimulatory effects of both nitrogen forms were observed for chlorophyll a- and diagnostic photopigment-based growth rates. In contrast, cell counts showed that only cryptophyte and dinoflagellate (Heterocapsa rotundata) growth were stimulated. Responses of other photopigments may have been due to an increase in pigment per cell or growth of plankton too small to be counted with the microscopic methods used. Despite high nitrate concentrations in August, growth rates were elevated in response to urea and/or nitrate addition for all photopigments except peridinin. However, this response was not observed in cell counts, again suggesting that pigment-based growth responses may not always be indicative of a true community and/or taxa-specific growth response. This highlights the need to employ targeted microscopy-based cell enumeration concurrent with pigment-based technology to facilitate a more complete understanding of phytoplankton dynamics in estuarine systems. These results are consistent with previous studies showing the seasonal importance of nitrogen availability in estuaries, and also reflect taxa-specific responses nitrogen availability. Finally, this study demonstrates that under nitrogen-limiting conditions, the phytoplankton community and its various taxa are capable of using both urea and nitrate to support growth.
Effect of hydrological variability on the biogeochemistry of estuaries across a regional climatic gradient
(Association for the Sciences of Limnology and Oceanography, 2018-08-15) Montagna, Paul A.; Hu, Xinping; Palmer, Terence A.; Wetz, Michael S.
Given projected changes in river flow to coastal regions worldwide due to climate change and increasing human freshwater demands, it is necessary to determine the role hydrology plays in regulating the biogeochemistry of estuaries. A climatic gradient exists along the Texas coast where freshwater inflow balance ranges from hydrologically positive to negative (where evaporation exceeds inflow) within a narrow latitudinal band, providing a natural experiment for examining inflow effects. Four Texas estuaries ranging from mesosaline to hypersaline were studied for 3 yr to determine how hydrological changes alter the biogeochemistry within and among the estuaries. Trends in dissolved inorganic nutrients, chlorophyll, dissolved organic matter, and carbonate chemistry indicated that these estuaries had drastically different biogeochemical signatures. Nutrients and chlorophyll patterns illustrated an emerging paradigm where phytoplankton biomass in positive estuaries is supported by “new” nitrogen from riverine input, while high concentrations of reduced nitrogen (organic, ammonium) allowed for high chlorophyll in the negative estuary. For carbonate chemistry, a positive estuary receiving river input from a limestone-dominated watershed was well-buffered under moderate to high freshwater inflow conditions. When weathering products were diluted during high-flow conditions, there is carbonate undersaturation (for aragonite) and decreases in pH. However, “acidification” was not observed in the negative estuary because evaporation concentrated the dissolved species and increased buffering capacity. Hydrological changes over spatial gradients are analogous to climatic changes over time, meaning climate change forecasts of higher temperatures and decreased precipitation can make the biogeochemistry of fresher estuaries change to the patterns of saltier estuaries.
Saharan dust deposition initiates successional patterns among marine microbes in the Western Atlantic
(Wiley Periodicals, Inc., 2019-08-02) Borchardt, Trace; Fisher, Kelsey V.; Ebling, Alina M.; Westrich, Jason R.; Xian, Peng; Holmes, Christopher D.; Landing, William M.; Lipp, Erin K.; Wetz, Michael S.; Ottesen, Elizabeth A.
Deposition of aerosolized desert dust can affect marine microbial community structure and function through pulsed addition of limiting micro- and macronutrients. However, few studies have captured responses to dust deposition in situ following trans-oceanic transport. We conducted a 26-d time series evaluating biogeochemical and microbial community response to Saharan dust deposition in surface waters in the subtropical western Atlantic (Florida Keys National Marine Sanctuary, U.S.A.). Following periods of elevated atmospheric dust concentrations, particulate and dissolved iron concentrations increased in surface waters. Autotrophic picoeukaryote abundance increased rapidly, followed by increases in the abundance of heterotrophic bacteria and Synechococcus. Concomitant to cell count changes, we observed successional shifts in bacterial community composition. The relative abundances of Prochlorococcus and Pelagibacter declined with dust arrival, while relative abundance of heterotrophic bacteria increased, beginning with Vibrionales and followed sequentially by Chrysophyceae, Rhodobacteriaceae, and Flavobacteriaceae. Finally, a peak in Synechococcus cyanobacteria was observed. These results provide new insight into microbial community succession in response to Saharan dust deposition, their association with temporal dynamics in surface water dissolved and particulate iron concentrations, and a potential role for bioprocessing of dust particles in shaping marine microbial responses to deposition events.
Water quality trends in Texas estuaries
(Elsevier Ltd., 2020-01-15) Bugica, Kalman; Sterba-Boatwright, Blair; Wetz, Michael S.
Coastal watersheds in Texas have experienced significant human population growth over the past several decades, yet there have been no comprehensive assessments of water quality trends in Texas estuaries. Here, analysis of historical estuarine water quality data indicates regional “hot spots” of change. Galveston Bay and Oso Bay, which have highly urbanized watersheds, currently exhibit symptoms of eutrophication. Symptoms of eutrophication were also found in the Baffin Bay-Upper Laguna Madre complex, which has a sparsely populated but agriculturally-intensive watershed. Increasing salinity was observed in estuaries of the central Texas coast and are attributed to long-term decreases in freshwater inflow. Another artifact of decreasing freshwater inflow is a reduction in the delivery of carbonate minerals to estuaries, which manifests as decreases in pH. With findings from this study, targeted studies can now be directed at the estuaries that are experiencing water quality degradation in order to guide future management efforts.
Photomineralization of organic carbon in a eutrophic, semiarid estuary
(Wiley Periodicals, Inc., 2020-01-28) Wang, Hongjie; Hu, Xinping; Wetz, Michael S.; Hayes, Kenneth C.; Lu, Kaijun
The effect of photomineralization on the carbon cycle in a eutrophic, semiarid estuary (Baffin Bay, Texas) was investigated using closed-system incubations. Photochemical production rate of dissolved inorganic carbon ranged from 0.16 to 0.68 μM hr−1, with a daily removal of 0.3∼1.5% of the standing stock of dissolved organic carbon (DOC). The photomineralization rate was negatively correlated with chlorophyll a concentration, suggesting that plankton-derived DOC was less photoreactive to solar radiation. The stable carbon isotope composition (δ13C∼ −18.6‰) of degraded DOC, as calculated using the DIC “Keeling” plot, further indicated high photochemical lability of 13C-enriched DOC in this semiarid environment. Our finding showed that photomineralization of 13C-enriched DOC is an important component of carbon cycle in this system, and this process does not necessarily remove 13C-depleted organic carbon as observed in other coastal systems.
Disparate Responses of Carbonate System in Two Adjacent Subtropical Estuaries to the Influence of Hurricane Harvey – A Case Study
(Frontiers in Marine Science, 2020-01-31) Hu, Xinping; Yao, Hongming; Staryk, Cory J.; McCutcheon, Melissa R.; Wetz, Michael S.; Walker, Lily
Two adjacent estuaries in the northwestern Gulf of Mexico (GOM) (Mission–Aransas or MAE and Guadalupe–San Antonio or GE), despite their close proximity and similar extents of freshening caused by Hurricane Harvey, exhibited different behaviors in their post-hurricane carbonate chemistry and CO2 fluxes. The oligotrophic MAE had little change in post-Harvey CO2 partial pressure (pCO2) and CO2 flux even though the center of Harvey passed right through, while GE showed a large post-Harvey increases in both pCO2 and CO2 flux, which were accompanied by a brief period of low dissolved oxygen (DO) conditions likely due to the large input of organic matter mobilized by the hurricane. The differences in the carbonate chemistry and CO2 fluxes were attributed to the differences in the watersheds from which these estuaries receive freshwater. The GE watershed is larger and covers urbanized areas, and, as a result, GE is considered relatively eutrophic. On the other hand, the MAE watershed is smaller, much less populous, and MAE is oligotrophic when river discharge is low. Despite that Harvey passed through MAE, the induced changes in carbonate chemistry and CO2 flux there were less conspicuous than those in GE. This study suggested that disturbances by strong storms to estuarine carbon cycle may not be uniform even on such a small spatial scale. Therefore, disparate responses to these disturbances need to be studied on a case-by-case basis.
Timescales and Magnitude of Water Quality Change in Three Texas Estuaries Induced by Passage of Hurricane Harvey
(Estuaries and Coasts, 2020-10-14) Walker, Lily M.; Montagna, Paul A.; Hu, Xinping; Wetz, Michael S.
Tropical cyclones represent a substantial disturbance to water quality in coastal ecosystems via storm surge, winds, and flooding. However, evidence to date suggests that the impacts of tropical cyclones on water quality are generally short-lived (days-months) and that the magnitude of the disturbance is related to proximity to storm track. Discrete and continuous water samples were collected in three Texas estuaries before and after Hurricane Harvey made landfall in 2017. Of the three estuaries, the Guadalupe Estuary and its watershed received the highest rainfall totals and wind speeds. An ephemeral increase in salinity was observed (mean of 9.8 on 24 August 2017 to a peak of 32.1 on 26 August 2017) due to storm surge and was followed by a rapid decrease to < 1 as floodwaters reached the estuary. Salinity returned to pre-storm levels within 1 month. During the low salinity period, bottom water hypoxia developed and lasted for 9 days. In all three estuaries, there was an increase in inorganic nutrients post-Harvey, but the nutrients largely returned to pre-storm baseline levels by winter. The lack of long-term water quality impacts from Harvey despite its severity corroborates previous findings that estuarine water quality tends to return to baseline conditions within days to a few months after storm passage.

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