Predicting changes in molluscan spatial distributions in mangrove forests in response to sea-level rise

Molluscs are an important component of the mangrove ecosystem, and the vertical distributions of molluscan species in this ecosystem are primarily dictated by tidal inundation. Thus, sea-level rise (SLR) may have profound effects on mangrove mollusc communities. Here, we used two dynamic empirical models based on measurements of surface elevation change, sediment accretion and zonation patterns of molluscs to predict changes in molluscan spatial distributions in response to different sea-level rise rates in the mangrove forests of Zhenzhu Bay (Guangxi, China). The change in surface elevation was 4.76–9.61 mm a during the study period (2016–2020), and the magnitude of surface-elevation change decreased exponentially as original surface elevation increased. Based on our model results, we predicted that mangrove molluscs might successfully adapt to a low rate of SLR (marker-horizon model: 2–4.57 mm a; plate model: 2–5.20 mm a) by 2100, with molluscs moving seaward and those in the lower intertidal zones expanding into newly available zones. However, as SLR rate increased (marker-horizon model: 4.57–8.14 mm a; plate model: 5.20–6.88 mm a), our models predicted that surface elevations would decrease beginning in the high intertidal zones and gradually spreading to the low intertidal zones. Finally, at high rates of SLR (marker-horizon model: 8.14–16.00 mm a; plate model: 6.88–16.00 mm a), surface elevations were predicted to decrease across the elevation gradient, with molluscs moving landward and species in higher intertidal zones would be blocked by landward barriers. Tidal inundation and the consequent increase in interspecific competition and predation pressure were predicted to threaten the survival of many molluscan groups in higher intertidal zones, especially species at the landward edge of the mangroves. Thus, future efforts to conserve mangrove floral and faunal diversity should prioritize species restricted to landward mangrove areas.

Advection Drives Nitrate Past the Microphytobenthos in Intertidal Sands, Fueling Deeper Denitrification

Nitrification rates are low in permeable intertidal sand flats such that the water column is the primary source of nitrate to the sediment. During tidal inundation, nitrate is supplied to the pore space by advection rather than diffusion, relieving the microorganisms that reside in the sand from nitrate limitation and supporting higher denitrification rates than those observed under diffusive transport. Sand flats are also home to an abundant community of benthic photosynthetic microorganisms, the microphytobenthos (MPB). Diatoms are an important component of the MPB that can take up and store high concentrations of nitrate within their cells, giving them the potential to alter nitrate availability in the surrounding porewater. We tested whether nitrate uptake by the MPB near the sediment surface decreases its availability to denitrifiers along deeper porewater flow paths. In laboratory experiments, we used NOx (nitrate + nitrite) microbiosensors to confirm that, in the spring, net NOx consumption in the zone of MPB photosynthetic activity was stimulated by light. The maximum potential denitrification rate, measured at high spatial resolution using microsensors with acetylene and nitrate added, occurred below 1.4 cm, much deeper than light-induced NOx uptake (0.13 cm). Therefore, the shallower MPB had the potential to decrease NOx supply to the deeper sediments and limit denitrification. However, when applying a realistic downward advective flow to sediment from our study site, NOx always reached the depths of maximum denitrification potential, regardless of light availability or season. We conclude that during tidal inundation porewater advection overwhelms any influence of shallow NOx uptake by the MPB and drives water column NOx to the depths of maximum denitrification potential.

Tidal monitoring on sandy beaches using perpendicular time-lapse photography

<p><span>Long term time-lapse photography has proven to be a key tool in monitoring changes in coastal environments, particularly in terms of morphology. The present study adapts and simplifies the approach of some precedents, such as the Argus and CoastSnap systems, to remotely monitor tidal inundation on a sandy beach at Magilligan on the north coast of Northern Ireland. Such a system could prove essential in the study of the effect of waves and tides on groundwater flow and saline intrusion in coastal aquifers, its consequences for sensitive subsurface infrastructure (such as water supply wells), and in the reconciliation of continuous data from same. Photographic data in this study have been gathered using a remote, solar powered time-lapse camera over a six-month period, capturing full neap and spring tidal cycles. Images are captured at hourly intervals and automatically uploaded to the cloud for remote access. The camera is located just 25 metres from the high water mark, overlooking the beach and perpendicular to the sea. This setup contrasts with previous studies where there is a need to find an elevated location at greater distance from the area of investigation. The extent to which a tide inundates up a sandy beach is governed primarily by astronomical effects, which are considered in this study, but also beach slope and atmospheric conditions. It is known that the beach at Magilligan has both a shallow grade (0.02 m) and a high tidal variation (> 150 m between spring and neap tides). Profiles of beach slope are gathered using a differential GPS, while a solar weather station on site, which also uploads data to the cloud, is used to gather atmospheric data. For tidal reference, a traditional tide gauge measuring tide levels at a pier 15 km east of the site is used. Captured images are post-processed using image analysis techniques based around characterising the tidal front against the visual contrast between pixels of sand and pixels of seawater using a routine in MATLAB</span><span><sup>®</sup></span><span>. From this analysis, a numerical value for tidal inundation is extracted. Analysis of these data indicates that the tide times (timing of high and low tides) correspond well with those measured at the nearby tide gauge, however important differences exist in terms of magnitude. In comparing these differences with atmospheric data from the site, it is possible to align larger and smaller inundation events with shifts in wind direction and speed. The calibration process involved in digitising the captured images is time-consuming, however, it may be possible to predict tidal inundation from a site using only a remote weather station — knowing how a change in wind speed or direction will affect inundation on the beach. It has already been shown that such instrumentation can be used to detect changes in beach morphology (as a key element in tidal inundation), this research therefore represents an important development in the low-cost remote monitoring of tidal inundation, particularly in locations where regular ground surveying is challenging. </span></p>

Land-sea interactions of tidal and sea breeze activity regulate mangrove carbon sink

<p>Coastal mangrove wetlands experience unique land-sea interactions including periodical tidal activity and land/sea breeze cycle. However, the influence of tidal and sea breeze activity on net ecosystem exchange of carbon dioxide (NEE) between mangrove and the atmosphere has not yet been investigated. In this study, temporal variations in mangrove-atmospheric NEE and its direct and indirect environmental controls were examined based on a three-year dataset of continuous eddy covariance and auxiliary measurements in a subtropical estuarine mangrove wetland of southeastern China. The results showed this mangrove wetland acted as a consistent carbon sink over the three-year period (mean NEE of -1233 g C m<sup>-2</sup> year<sup>-1</sup>) with the strongest carbon sink capacity in spring, and the impacts of environmental factors on mangrove NEE varied across time scales: (1) half-hourly daytime carbon influx was regulated by photosynthetically active radiation (PAR) with down-regulation effects from high temperature and vapor pressure deficit (VPD), while half-hourly nighttime carbon efflux was dominated by air temperature with additional suppression effects from tidal inundation and rain; (2) the importance of environmental factors in controlling daily NEE decreased in the order of PAR, air temperature, sea breeze, VPD, tidal salinity, and tidal inundation; (3) the seasonality of monthly NEE was strongly regulated by tidal inundation and rain. This was the first study to examine both direct and indirect effects of tidal and sea breeze activity on mangrove NEE using long-term continuous eddy covariance measurements, and to confirm the importance of previously neglected indirect effects of tidal and sea breeze on mangrove carbon sink. Strong negative correlations between mangrove carbon sink and air temperature/tidal inundation implied that mangrove wetland could become a weaker blue carbon sink in response to global warming and sea level rise in the future.</p>