6 research outputs found
Sea-level rise will drive divergent sediment transport patterns on fore reefs and reef flats, potentially causing erosion on Atoll Islands
Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Earth Surface 125 (2020): e2019JF005446, doi: 10.1029/2019JF005446.Atoll reef islands primarily consist of unconsolidated sediment, and their oceanâfacing shorelines are maintained by sediment produced and transported across their reefs. Changes in incident waves can alter crossâshore sediment exchange and, thus, affect the sediment budget and morphology of atoll reef islands. Here we investigate the influence of sea level rise and projected wave climate change on wave characteristics and crossâshore sediment transport across an atoll reef at Kwajalein Island, Republic of the Marshall Islands. Using a phaseâresolving model, we quantify the influence on sediment transport of quantities not well captured by waveâaveraged models, namely, wave asymmetry and skewness and flow acceleration. Model results suggest that for current reef geometry, sea level, and wave climate, potential bedload transport is directed onshore, decreases from the fore reef to the beach, and is sensitive to the influence of flow acceleration. We find that a projected 12% decrease in annual wave energy by 2100 CE has negligible influence on reef flat hydrodynamics. However, 0.5â2.0 m of sea level rise increases wave heights, skewness, and shear stress on the reef flat and decreases wave skewness and shear stress on the fore reef. These hydrodynamic changes decrease potential sediment inputs onshore from the fore reef where coral production is greatest but increase potential crossâreef sediment transport from the outer reef flat to the beach. Assuming sediment production on the fore reef remains constant or decreases due to increasing ocean temperatures and acidification, these processes have the potential to decrease net sediment delivery to atoll islands, causing erosion.This study was supported by the Strategic Environmental Research and Development Program through awards SERDP: RCâ2334, and RCâ2336. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.2021-03-2
The role of pH up-regulation in response to nutrient-enriched, low-pH groundwater discharge
Highlights
âą Dual geochemical approach using ÎŽ11B and B/Ca to evaluate coral calcifying fluids from West Maui, Hawai'i.
âą NMR analysis confirms boron is present as borate with no evidence of boric acid inclusion.
âą Increased pH up-regulation in corals exposed to high nutrient / low pH submarine groundwater discharge.
âą Calcifying fluid aragonite saturate state 9 to 10 times higher than ambient seawater.
âą Up-regulation as an internal coping mechanism to combat multiple stressors from land-based sources of pollution.
Coral reefs and their ecosystems are threatened by both global stressors, including increasing sea-surface temperatures and ocean acidification (OA), and local stressors such as land-based sources of pollution that can magnify the effects of OA. Corals can physiologically control the chemistry of their internal calcifying fluids (CF) and can thereby regulate their calcification process. Specifically, increasing aragonite saturation state in the CF (ΩCF) may allow corals to calcify even under external low saturation conditions. Questions remain regarding the physiological processes that govern the CF chemistry and how they change in response to multiple stressors. To address this knowledge gap, the boron systematics (Ύ11B and B/Ca) were analyzed in tropical corals, Porites lobata, collected at submarine groundwater seeps impacted by the release of treated wastewater in west Maui, Hawai'i, to document the interactions between high nutrient / low pH seep water on CF carbonate chemistry. Results show substantial up-regulation of pH and dissolved inorganic carbon (DIC) with respect to seawater in P. lobata corals collected from within the wastewater impacted area at Kahekili Beach Park compared to the control site at Olowalu Beach. The ΩCF was 9 to 10 times higher than ambient seawater Ω, and 13 to 26% higher than in corals from the control site and from previously observed in tropical Porites spp. corals. Such elevated up-regulation suggests that corals exposed to nutrient-enriched, low pH effluent sustain CF supersaturated with respect to aragonite, possibly as an internal coping mechanism to combat multiple stressors from land-based sources of pollution. This elevated up-regulation has implications to coral vulnerability to future climate- and ocean-change scenarios
Suspended particulate layers and internal waves over the southern Monterey Bay continental shelf: An important control on shelf mud belts?
The article of record as published may be located at http://dx.doi.org/10.1002/2013JC009360Physical and optical measurements taken over the mud belt on the southern continental
shelf of Monterey Bay, California documented the frequent occurrence of suspended
particulate matter features, the majority of which were detached from the seafloor, centered
9â33 m above the bed. In fall 2011, an automated profiling mooring and fixed
instrumentation, including a thermistor chain and upward-looking acoustic Doppler current
profiler, were deployed at 70 m depth for 5 weeks, and from 12 to 16 October a long-range
autonomous underwater vehicle performed across-shelf transects. Individual SPM events
were uncorrelated with local bed shear stress caused by surface waves and bottom currents.
Nearly half of all observed SPM layers occurred during 1 week of the study, 9â16 October
2011, and were advected past the fixed profiling mooring by the onshore phase of
semidiurnal internal tide bottom currents. At the start of the 9â16 October period, we
observed intense near-bed vertical velocities capable of lifting particulates into the middle
of the water column. This ââupdraftââ event appears to have been associated with nonlinear
adjustment of high-amplitude internal tides over the mid and outer shelf. These findings
suggest that nonlinear internal tidal motions can erode material over the outer shelf and that,
once suspended, this SPM can then be transported shoreward to the middle and shallow
sections of the mud belt. This represents a fundamental broadening of our understanding of
how shelf mud belts may be built up and sustained.This project was funded by the National Science Foundation (grant OCE0961810 to McPhee-Shaw, Bellingham, Shaw, and Stanton), and by the U.S. Geological Survey
Upwelling rebound, ephemeral secondary pycnoclines, and the creation of a near-bottom wave guide over the Monterey Bay continental shelf
The article of record as published may be found at http://dx.doi.org/10.1002/2014GL061897The USGS
data sets presented herein can be
obtained by sending a written request
to the corresponding author.Several sequential upwelling events were observed in fall 2012, using measurements from
the outer half of the continental shelf in Monterey Bay, during which the infiltration of dense water onto
the shelf created a secondary, near-bottom pycnocline. This deep pycnocline existed in concert with the
near-surface pycnocline and enabled the propagation of near-bottom, cold, semidiurnal internal tidal bores,
as well as energetic, high-frequency, nonlinear internal waves of elevation (IWOE). The IWOE occurred within
20m of the bottom, had amplitudes of 8â24 m, periods of 6â45 min, and depth-integrated energy fluxes up
to 200Wm 1. Iribarren numbers (<0.03) indicate that these IWOE were nonbreaking in this region of the
shelf. These observations further demonstrate how regional upwelling dynamics and the resulting bulk,
cross-margin hydrography is a first-order control on the ability of internal waves, at tidal and higher
frequencies, to propagate through continental shelf waters.U.S. Geological Surveyâs Coastal and Marine Geology Program.National Science FoundationNational Science Foundation grant OCE096181
Internal tides can provide thermal refugia that will buffer some coral reefs from future global warming
Abstract Observations show ocean temperatures are rising due to climate change, resulting in a fivefold increase in the incidence of regional-scale coral bleaching events since the 1980s; analyses based on global climate models forecast bleaching will become an annual event for most of the worldâs coral reefs within 30â50 yr. Internal waves at tidal frequencies can regularly flush reefs with cooler waters, buffering the thermal stress from rising sea-surface temperatures. Here we present the first global maps of the effects these processes have on bleaching projections for three IPCC-AR5 emissions scenarios. Incorporating semidiurnal temperature fluctuations into the projected water temperatures at depth creates a delay in the timing of annual severe bleachingââ„â10 yr (â„â20 yr) for 38% (9%), 15% (1%), and 1% (0%) of coral reef sites for the low, moderate, and high emission scenarios, respectively; regional averages can reach twice as high. These cooling effects are greatest later in twenty-first century for the moderate emission scenarios, and around the middle twenty-first century for the highest emission scenario. Our results demonstrate how these effects could delay bleaching for corals, providing thermal refugia. Identification of such areas could be a factor for the selection of coral reef marine protected areas