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Climate change is causing decreases in pH and dissolved oxygen (DO) in coastal ecosystems. Canopy-forming giant kelp can locally increase DO and pH through photosynthesis, with the most pronounced effect expected in surface waters where the bulk of kelp biomass resides. However, limited observations are available from waters in canopies and measurements at depth show limited potential of giant kelp to ameliorate chemical conditions. We quantified spatiotemporal variability of surface biogeochemistry and assessed the role of biological and physical drivers in pH and DO modification at two locations differing in hydrodynamics inside and outside of two kelp forests in Monterey Bay, California in summer 2019. pH, DO, dissolved inorganic carbon (DIC), and temperature were measured at and near the surface, in conjunction with physical parameters (currents and pressure), nutrients, and metrics of phytoplankton and kelp biological processes. DO and pH were highest, with lower DIC, at the surface inside kelp forests. However, differences inside vs. outside of kelp forests were small (DO 6–8%, pH 0.05 higher in kelp). The kelp forest with lower significant wave height and slower currents had greater modification of surface biogeochemistry as indicated by larger diel variation and slightly higher mean DO and pH, despite lower kelp growth rates. Differences between kelp forests and offshore areas were not driven by nutrients or phytoplankton. Although kelp had clear effects on biogeochemistry, which were modulated by hydrodynamics, the small magnitude and spatial extent of the effect limits the potential of kelp forests to mitigate acidification and hypoxia. 

The interaction of coral reefs, both chemically and physically, with the surrounding seawater is governed, at the smallest scales, by turbulence. Here, we review recent progress in understanding turbulence in the unique setting of coral reefs?how it influences flow and the exchange of mass and momentum both above and within the complex geometry of coral reef canopies. Flow above reefs diverges from canonical rough boundary layers due to their large and highly heterogeneous roughness and the influence of surface waves. Within coral canopies, turbulence is dominated by large coherent structures that transport momentum both into and away from the canopy, but it is also generated at smaller scales as flow is forced to move around branches or blades, creating wakes. Future work interpreting reef-related observations or numerical models should carefully consider the influence that spatial variation has on momentum and scalar flux. 

We discuss observations of tidally varying wave‐forced flows in the reef system on Ofu, American Samoa, a barrier reef and lagoon system that appears open at low tide and closed at high tide. At high tide, the free‐surface pressure gradient nearly balances the radiation stress gradient in the depth‐integrated momentum equation. At depth, there is an imbalance between these two forces, generating an undertow and flows that turn alongshore, and for some of the time, offshore, behavior similar to rip currents observed on beaches. At low tides, the wave forcing drives purely onshore flows. In general, wave transport is important to determining the total net transport. While the dynamically closed nature of the lagoon mostly suppresses cross‐reef transport, there is always some flow through the lagoon with the strongest flows occurring at high tides and when the wave forcing is strongest.

 

As part of a project focused on the coastal fisheries of Isla Natividad, an island on the Pacific coast of Baja California, Mexico, we conducted a 2‐1/2 year study of flows at two sites within the island's kelp forests. At one site (Punta Prieta), currents are tidal, whereas at the other site (Morro Prieto), currents are weaker and may be more strongly influenced by wind forcing. Satellite estimates of the biomass of the giant kelp (Macrocystis pyrifera) for this period varied between 0 (no kelp) and 3 kg/m²(dense kelp forest), including a period in which kelp entirely was absent as a result of the 2014–2015 “Warm Blob” in the Eastern Pacific. During this natural “deforestation experiment”, alongshore velocities at both sites when kelp was present were substantially weaker than when kelp was absent, with low‐frequency alongshore currents attenuated more than higher frequency ones, behavior that was the same at both sites despite differences in forcing. The attenuation of cross‐shore flows by kelp was less than alongshore flows; thus, residence times for water inside the kelp forest, which are primarily determined by cross‐shore velocities, were only weakly affected by the presence or absence of kelp. The flow changes we observed in response to changes in kelp density are important to the biogeochemical functioning of the kelp forest in that slower flows imply longer residence times, and, are also ecologically relevant in that reduced tidal excursions may lead to more localized recruitment of planktonic larvae.

 

Shallow dynamic flows are very important processes in environmental systems, yet they are notoriously difficult to measure. For example, on coral reefs, shallow flows over the reef crest typically range from 0 to 1 m depth and velocity up to 1–2 m s⁻¹including oscillatory motion of waves. To directly measure depth and velocity in this challenging environment, we retrofitted a vertical acoustic Doppler current profiler (ADCP, Teledyne, RDI) designed for streams into a new shallow water acoustic Doppler current profiler system (SW‐ADCP) to expand memory and power limitations and allow for deployment in wave‐dominated environments. We captured variations in shallow reef crest depth and flow over a period of 3 weeks in the reef/lagoon system of Ofu Island, American Samoa. The new SW‐ADCPs recorded water depth and three‐dimensional velocity in 3 cm bins every 3 s. We then used velocity profiles to estimate volumetric flow and drag coefficients and verified these estimates using boundary layer theory. We observe that the mean velocity profile is well approximated by a log‐layer formulation withz₀of 3.5 cm, despite the shallow depths, strong flows, and breaking waves. Our observations validated the use of SW‐ADCPs as a tool for measuring flows in shallow (0.1–1 m), dynamic coastal marine environments.

 

We report direct measurement of drag forces due to tidal flow over a submerged seagrass bed in Ngeseksau Reef, Koror State, Republic of Palau. In our study, drag is computed using an array of high‐resolution pressure measurements, from which values of the drag coefficients,C_D, referenced to measured depth‐averaged velocities,V, were inferred. Reflecting the fact that seagrass blades deflect in the presence of flow, we find thatC_Dis O(1) when flows are weak and tends toward a value of 0.03 at the highest velocities, behavior that is consistent with existing theory for canopy flows with flexible canopy elements. A limited subset of velocity profiles obey the law of the wall, producing values of shear velocity that, while noisy, broadly agree with values inferred from the pressure measurements.

 

Strong and sustained winds can drive dramatic hydrodynamic responses in density‐stratified lakes, with the associated transport and mixing impacting water quality, ecosystem function, and the stratification itself. Analytical expressions offer insight into the dynamics of stratified lakes during severe wind events. However, it can be difficult to predict the aggregate response of a natural system to the superposition of hydrodynamic phenomena in the presence of complex bathymetry and when forced by variable wind patterns. Using an array of current, temperature, and water quality measurements at the upwind shore, we detail the hydrodynamic response of deep, rotationally influenced Lake Tahoe to three strong wind events during late spring. Sustained southwesterly winds in excess of 10 m s⁻¹drove upwelling at the upwind shore (characteristic of non‐rotational upwelling setup), with upward excursions of deep water exceeding 70 m for the strongest event. Hypolimnetic water, with elevated concentrations of chlorophyllaand nitrate, was advected toward the nearshore, but this water rapidly returned to depth with the relaxation of upwelling after the winds subsided. The relaxation of upwelling exhibited rotational influence, highlighted by an along‐shore, cyclonic front characteristic of a Kelvin wave‐driven coastal jet, with velocities exceeding 25 cm s⁻¹. The rotational front also produced downwelling to 100 m, transporting dissolved oxygen to depth. More complex internal wave features followed the passage of these powerful internal waves. Results emphasize the complexity of these superimposed hydrodynamic phenomena in natural systems, providing a conceptual reference for the role upwelling events may play in lake ecosystems.