Spatial variation and availability of nutrients at an oyster reef in relation to submarine groundwater discharge

Submarine groundwater discharge interacts with creek geomorphology to affect eastern oyster Crassostrea virginica growth rates in a coastal Georgia creek

Eastern oysters, Crassostrea virginica, are commercially important coastal species that provide many ecosystem services for coastal communities. Unfortunately, 85% of oyster reefs have been lost globally, prompting investments in restoration efforts to rebuild populations. Managers often consider several well-studied environmental and water quality parameters when making restoration site decisions. However, recent research suggests that submarine groundwater discharge (SGD) may play a role in driving the distribution of oysters in some estuaries. Specifically, SGD may result in localized areas of low dissolved oxygen and low pH that could inhibit oyster recruitment and survival. However, SGD may interact with other potential oyster stressors, including creek geomorphology. On point bars, sediment accumulation could alter growth rates of oysters and physiology, and it is possible that the two factors, SGD and creek geomorphology, could interact to impact oyster growth. We conducted a field experiment to examine the effects of SGD and creek geomorphology on oyster growth rates in a marsh-lined tidal creek in Georgia, USA. High and low SGD sites were paired within point bars and cut banks. Oysters were deployed in cages for 72 days and growth rates were determined. We found a significant interaction between SGD and creek geomorphology on oyster growth rates. Oysters grew at significantly faster rates at locations on accretionary point bars regardless of SGD flux, whereas, on erosional cut banks, high SGD flux significantly reduced oyster growth rate relative to low SGD flux. It appears that SGD may negatively influence oyster growth at specific creek locations, likely due to the presence of other stressors. Therefore, it is important to consider potential interacting and confounding stressors when managing oyster populations. As SGD is still a relatively understudied potential stressor for oysters, it is critical to continue to examine how groundwater might influence oysters in other locations and in combination with other stressors. Regardless, this study provides further evidence that SGD should be considered in future management efforts.

Natural heavy metal concentrations in seawater as a possible cause of low survival of larval mussels

BACKGROUND

A period of seismic activity starting in 2010 coincided with a decline in commercial catches of wild seed mussels in a major aquaculture production region of New Zealand. Analyses of over 40 years of mussel seed catch data from in the Pelorus and Kenepuru Sounds, confirmed a marked decline since 2010 in catches of the preferred, green-lipped mussel (Perna canaliculus), the larvae of which is known to have low tolerance of heavy metals in seawater.

METHODS

Heavy metal mean concentrations were measured throughout the Pelorus and Kenepuru Sounds. The concentrations ranged from < 0.60-3.24, < 16.94-74.35, < 1.47-4.00, 2.23-19.02, 1.86-3.29 and 0.12-0.52 µg L^-1 for Cr, Fe, Cu, Zn, As, and Cd, respectively. Seawater from six locations in the Sounds, historically associated with high commercial catches of settling mussel larvae, was used for experimental rearing of green-lipped mussel larvae.

RESULTS

No mussel embryos survived when incubated in these seawater samples. The mean concentrations of Cr, Fe, As, and Cd were significantly higher in the seawater from the Sounds than in the hatchery seawater. A higher concentration of one or a combination of these heavy metals could be the cause of the poor larval survival. These findings could be crucial for the sustainability of mussel farming in the area.

Subtropical estuarine carbon budget under various hydrologic extremes and implications on the lateral carbon exchange from tidal wetlands

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.

Artificial Intelligence Search Strategies for Autonomous Underwater Vehicles Applied for Submarine Groundwater Discharge Site Investigation

In this study, a set of different search strategies for locating submarine groundwater discharge (SGD) are investigated. This set includes pre-defined path planning (PPP), adapted random walk (RW), particle swarm optimisation (PSO), inertia Levy-flight (ILF), self-organising-migration-algorithm (SOMA), and bumblebee search algorithm (BB). The influences of self-localisation and communication errors and limited travel distance of the autonomous underwater vehicles (AUVs) on the performance of the proposed algorithms are investigated. This study shows that the proposed search strategies could not outperform the classic search heuristic based on full coverage path planning if all AUVs followed the same search strategy. In this study, the influence of self-localisation and communication errors was investigated. The simulations showed that, based on the median error of the search runs, the performance of SOMA was in the same order of magnitude regardless the strength of the localisation error. Furthermore, it was shown that the performance of BB was highly affected by increasing localisation errors. From the simulations, it was revealed that all the algorithms, except for PSO and SOMA, were unaffected by disturbed communications. Here, the best performance was shown by PPP, followed by BB, SOMA, ILF, PSO, and RW. Furthermore, the influence of the limited travel distances of the AUVs on the search performance was evaluated. It was shown that all the algorithms, except for PSO, were affected by the shorter maximum travel distances of the AUVs. The performance of PPP increased with increasing maximum travel distances. However, for maximum travel distances > 1800 m the median error appeared constant. The effect of shorter travel distances on SOMA was smaller than on PPP. For maximum travel distances < 1200 m, SOMA outperformed all other strategies. In addition, it can be observed that only BB showed better performances for shorter travel distances than for longer ones. On the other hand, with different search strategies for each AUV, the search performance of the whole swarm can be improved by incorporating population-based search strategies such as PSO and SOMA within the PPP scheme. The best performance was achieved for the combination of two AUVs following PPP, while the third AUV utilised PSO. The best fitness of this combination was 15.9. This fitness was 26.4% better than the performance of PPP, which was 20.4 on average. In addition, a novel mechanism for dynamically selecting a search strategy for an AUV is proposed. This mechanism is based on fuzzy logic. This dynamic approach is able to perform at least as well as PPP and SOMA for different travel distances of AUVs. However, due to the better adaptation to the current situation, the overall performance, calculated based on the fitness achieved for different maximum travel distances, the proposed dynamic search strategy selection performed 32.8% better than PPP and 34.0% better than SOMA.

Hydroclimatic variability drives submarine groundwater discharge and nutrient fluxes in an anthropogenically disturbed, semi-arid estuary

Nutrient budgets in semi-arid estuaries, with ephemeral freshwater inflows and limited nutrient sources, are likely incomplete if contributions from submarine groundwater discharge (SGD) are not included. Here, the relative importance of saline/recirculated SGD-derived nutrient fluxes spatiotemporal variability to the overall nutrient budget is quantified for Nueces Bay, Texas, U.S.A., across hydroclimatic conditions ranging from drought to normal, to flood. On average, 67% of the variance in water quality is due to temporal differences while 16% is explained by spatial differences. Principal component analysis (PCA) reveals three principal components: freshwater inflow (PC1 28.8%), saline/recirculated SGD and recycled nitrogen (PC2 15.6%), and total SGD and "new" nitrogen (PC3 11.2%). Total SGD porewater fluxes ranged from 29.9-690.3 mmol∙m^-2d^-1 for ammonium, 0.21-18.7 mmol∙m^-2d^-1 for nitrite+nitrate, 3.1-51.3 mmol∙m^-2d^-1 for phosphate, 57.1-719.7 mmol∙m^-2d^-1 for silicate, and 95.9-36,838.5 mmol∙m^-2d^-1 for dissolved organic carbon. Total and saline/recirculated SGD fluxes were on average 150-26,000 and 5.8-466 times, respectively, greater than surface runoff fluxes across all seasons. Nitrogen (N) enrichment in porewater occurs near the agricultural fields because of soil N flushing and percolation to groundwater, which facilitates N-rich groundwater fluxes. There were substantial "new" N inputs from terrestrial groundwater following precipitation while saline/recirculated SGD of recycled N accounts for only <4% of total SGD inputs. The "new" N inputs occur in the river and river mouth during flooding, and near the north shore where topography and hydraulic gradients are steeper during drought. Thus, while significant inputs of N may be associated with atmospheric deposition, or remineralization in the porewater, groundwater is the highest contributor to the nutrient budget in Nueces Bay. This result implies that nutrient management strategies should focus on land-use practices to reduce N contamination of shallow groundwater and subsequent contamination of estuaries.

Evaluation, effect and utilization of submarine groundwater discharge for coastal population and ecosystem: A special emphasis on Indian coastline

Submarine groundwater discharge (SGD) is an important process driven by marine and terrestrial forces. Low tide affects SGD the most, therefore the ideal time to detect SGD is the low tide, especially during spring tide. Techniques to detect and quantify SGD along with the understanding of the related aquifer characteristics is discussed in this study. Scientific community across the world is realizing the importance of studying and mapping SGD because in the scenario of climate change, this part of the global hydrological cycle is an important process and is known to have a significant effect on the marine ecosystem due to nutrient and metal inputs around the region of discharge. Therefore, understanding the processes governing SGD becomes very important. In this review, various components and processes related to SGD (e.g. Submarine Groundwater Recharge, Deep Porewater Upwelling, Recirculated Saline Groundwater Discharge), along with detailed discussion on impacts of SGD for marine ecosystem is presented. Also, it highlights the future research direction and emphasis is put on more research to be done keeping in mind the changing climate and its impacts on SGD.