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We present an approach for estimating drag coefficients for depth-averaged tidal flows that uses the ratio of observed RMS velocities to the RMS velocities that would be observed without bottom friction. We find that this ratio, R, depends on a single non-dimensional number, P = C D C η 0 / ω H 2 , where CD is the drag coefficient, and C is the phase speed of a tidal wave with amplitude η 0 and frequency ω , in water of depth h. The function R(P) can be inverted to solve for CD using measured values of R. Taking advantage of a unique multi-year record of tidal flows on Isla Nativdad, Baja California, Mexico, during which time the kelp forest there varied between non-existent and dense, we use this method to quantify the effect of kelp biomass on drag. This analysis shows that a maximum value of CD ≈ 0.04 is reached for relatively low values of kelp biomass, which may be an effect of sheltering (reductions in the velocity creating drag due to the close proximity of bundles of kelp stipes). However, values as large as 0.015 were observed when the water column experienced strong secondary flows in the presence of strong density stratification. Given that the long-term measurements were made near a coastal headland, we argue that this may reflect variations in secondary flow strength due to stratification. Lastly, our measurements show little evidence of enhancement of drag by surface waves. [ABSTRACT FROM AUTHOR]

We report a large bloom of a benthic diatom, Biddulphia biddulphiana, in the Midriff Islands region of the Gulf of California (GOC), Mexico. The bloom is seemingly invasive in nature (abnormal spatial domination of the benthic environment), and no similar blooms have been previously described in the GOC. Quantitative results indicate a non-significant increase in density and reveal an expansion of affected area from "ground zero" on Isla San Esteban during the year after first observance (2009 to 2010). Observational evidence from colleagues suggests that the bloom had completely disappeared from the areas of highest density by June 2011. It is unclear how a seemingly growing bloom can retract so significantly in such a short time. [ABSTRACT FROM AUTHOR]

n idealized model for tide propagation and amplification in semi-enclosed rectangular basins is presented, accounting for depth differences by a combination of longitudinal and lateral topographic steps. The basin geometry is formed by several adjacent compartments of identical width, each having either a uniform depth or two depths separated by a transverse topographic step. The problem is forced by an incoming Kelvin wave at the open end, while allowing waves to radiate outward. The solution in each compartment is written as the superposition of (semi)-analytical wave solutions in an infinite channel, individually satisfying the depth-averaged linear shallow water equations on the f plane, including bottom friction. A collocation technique is employed to satisfy continuity of elevation and flux across the longitudinal topographic steps between the compartments. The model results show that the tidal wave in shallow parts displays slower propagation, enhanced dissipation and amplified amplitudes. This reveals a resonance mechanism, occurring when the length of the shallow end is roughly an odd multiple of the quarter Kelvin wavelength. Alternatively, for sufficiently wide basins, also Poincaré waves may become resonant. A transverse step implies different wavelengths of the incoming and reflected Kelvin wave, leading to increased amplitudes in shallow regions and a shift of amphidromic points in the direction of the deeper part. Including the shallow parts near the basin's closed end (thus capturing the Kelvin resonance mechanism) is essential to reproduce semi-diurnal and diurnal tide observations in the Gulf of California, the Adriatic Sea and the Persian Gulf. [ABSTRACT FROM AUTHOR]

Current and sea level measurement data from three ∼75-day deployments at the inlet and the inner shelf of the Caimanero Lagoon were used with data of wind, evaporation, and river discharge to describe the seasonal subtidal exchange in the shelf-lagoon system. During the dry season, the estuary mouths were partially closed by the littoral drift; however, at the shelf, the oceanic mean sea level rose because of a steric response to advection of warm water on the continental shelf. In addition, the annual oscillation reached its maximum in August. Both events induced a barotropic flow of sea water lagoonward. Dry season currents were 8-10 cm s-1. During the wet season, estuary mouths were reopened as result of river discharges, which caused a barotropic flow toward the sea. During this period, river transport occurred at times scales of 2∼10 days; the magnitude of the transport was larger than that caused by the sea level. Wet season currents were 10-40 cm s-1. The observation that this barotropic flow occurred during both seasons is the principal discovery of this work and demonstrates that the barotropic motion is an important component of net circulation. The shelf circulation during the dry season showed a two layer system with a cross-shore flow toward the shore with velocities of about ±5-10 cm s-1, while longshore current flow was southward on the surface and northward (up gulf) below 15 m. This current pattern apparently contributes to the transport of shrimp larvae to the shore, acting as the driving mechanism, knowing that shrimp postlarvae in the area appear in June and disappear in middle August. During the wet season, on the other hand, inner shelf circulation shows currents of mainly barotropic flow that respond to hurricanes with maximum velocities of about 40 cm s-1. [ABSTRACT FROM AUTHOR]

Abstract The relative importance of factors that may promote genetic differentiation in marine organisms is largely unknown. Here, contributions to population structure from a biogeographic boundary, geographical distance and the distribution of suitable habitat were investigated in Axoclinus nigricaudus, a small subtidal rock-reef fish, throughout its range in the Gulf of California. A 408-bp fragment of the mitochondrial control region was sequenced from 105 individuals. Variation was significantly partitioned between 28 of 36 possible combinations of population pairs. Phylogenetic analyses, hierarchical analyses of variance and a modified Mantel test substantiated a major break between two putative biogeographic regions. This genetic discontinuity coincides with an abrupt change in ecological characteristics, including temperature and salinity, but does not coincide with known oceanographic circulation patterns or any known historic barriers. There was an overall relationship of increasing genetic distance with increasing geographical distance between population pairs, in a manner consistent with isolation-by-distance. A significant habitat-by-geographical-distance interaction term indicated that, for a given geographical distance, populations separated by discontinuous habitat (sand) are more distinct genetically than are populations separated by continuous habitat (rock). In addition, populations separated by deep open waters were more genetically distinct than populations separated by continuous habitat (rock). These results indicate that levels of genetic differentiation among populations of A. nigricaudus cannot be explained by a single factor, but are due to the combined influences of biogeography, geographical distance and availability of suitable habitat. [ABSTRACT FROM AUTHOR]