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Sea level rise and atoll reef dynamics
Sea level rise and atoll reef dynamics
In a newly published paper, we study how ocean waves and tides move water across shallow coral reefs in the southern Huvadhu Atoll, Maldives, during a four-month field project. When waves break on the outer edge of the reef, they push water over the reef flat toward the lagoon. This movement depends on how deep the water is and how strong the waves are. We find a tipping point: if the reef is shallow enough, waves keep breaking and drive strong flows toward the lagoon. However, if the reef is too deep, waves stop breaking, and the flow weakens. This helps us study how sea-level rise could change reef and atoll conditions - leading to more frequent periods with little or no flow, and less wave-driven water movement across reef flats.
Short wave attenuation by kelp forest canopies
Short wave attenuation by kelp forest canopies
Giant kelp is an ubiquitous macro alga found along the shores of the California current and in both the northern and southern hemispheres. This fast-growing alga can span the water column, growing large, buoyant canopies that occupy the sea surface, and can form “forests” that are several kilometers in horizontal extent. These kelp forests provide unique shelter and habitat for many marine species, and can significantly modify the physical and chemical environment around them. Here, we studied the influence of a large kelp forest on ocean surface waves. we find that the presence of giant kelp surface canopy reduces short wave energy, an effect that is comparable to that of sea ice.
The attenuating effect of the kelp forest canopy was evident at two temporal scales, peaking during summer months and low tide, where the canopy reached maximum vertical and horizontal extent.
Read our paper here: https://doi.org/10.1002/lol2.10401
Reef roughness and residence times
Reef roughness and residence times
Through idealized, numerical models this work investigates flows on a reef geometry which has received significant attention in the literature; a shallow, fringing reef with deeper, shore-ward pools or lagoons. This paper shows that reef roughness (and thus reef health) plays a significant role in determining circulation patterns and residence times. A rougher, healthier reef has a shorter residence time and more circulation than a less rough, unhealthy reef. Thus, loss of reef roughness could have implications for transport and mixing of nutrients and water masses, as well as larval dispersal.
Wave-driven flows
Wave-driven flows
Reefs are incredibly diverse in terms of biology, ecology, physical environment, and geometry. We set up numerical models of two typical types of reef geometries, barrier reefs and fringing reefs, and investigated how the forcing and flow on the reefs varied with tides and incident wave heights. Depending on forcing, the reefs behaved more open channel-like ('open reefs') or beach-like ('closed reefs'), and open reefs had stronger flow and water renewal than closed reefs. The figure above is from our paper, where we summarize characteristics of the two dynamical regimes. The paper is inspired by observations from Ofu, American Samoa, a reef that seems to transition between open and closed depending on wave and tidal conditions, making it particularly interesting.
Reef boundary layers
Reef boundary layers
We are combining 3D reef models from Structure-from-Motion with observations from acoustic Doppler current profilers (ADCPs) to understand the spatial homogeneity of reef boundary layers. Field work is always limited by time and resources, so how representative are our measurements? If our instruments were displaced slightly on the reef, would we get the same results? Results from a reef in the Chagos archipelago show surprising agreement between different types of instruments distributed over the reef, see the video above. Here the velocity measurements from three instruments are superimposed on the reef bathymetry, and all instruments are capturing the same overall flow structure in the water column. This is encouraging news for scientists working in the field! Read our paper here, a press release here, and watch a short video here.
Reef monitoring
Reef monitoring
Combining estimates of flow and turbulence with biogeochemical measurements, our colleagues at Stanford Earth System Science can estimate reef metabolism rates and take the pulse of the world's reefs. The photo shows a setup in Chagos, where we deployed these systems on three different reefs in 2019. In collaboration with colleagues from the Zoological Society of London we are carrying out an interdisciplinary study combining results from physical oceanography, chemistry, and genomics to understand the influence of environmental conditions on benthic communities and coral recruitment across the three reefs.
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