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Velocity and hydrography measurements were used to determine the tidal variability and detailed structure of an intense (< 43,200 m 3 d 21 ) point-source submarine groundwater discharge from a spring located in the coastal ocean of the Yucatá n peninsula, Mexico. Measurements were obtained during a dry season with a combination of towed, profiling, and fixed instrumentation. The goal of these observations was to understand the effects of tides on the velocity and salinity structure of the water column that determine mixing and dispersion processes at spatial scales from meters to kilometers. Tidally averaged flows were characterized by an upward jet flanked by asymmetric downdrafts. The asymmetry was caused by a < 0.1 m s 21 ambient horizontal flow and by the < 40u angle (relative to the vertical) of discharge. The spatially averaged salinity distributions exhibited lowest values at low tides and highest values at high tides. This was attributed to tidal variations of hydrostatic pressure acting on the spring outflow, which allowed , 1 m s 21 vertical flows at low tides and , 0.05 m s 21 vertical velocities at high tides. The vertical momentum balance consisted of upward accelerations produced by buoyancy and inertia from the spring and downward accelerations from gravity. On the basis of this balance and of energetic considerations, the vertical extent of the spring discharge likely depends on the density contrast between ambient and discharge waters (buoyancy forces), the upward speed of the spring (inertia forces), and the depth of the water column (hydrostatic pressure force), all of which vary with tides.

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Background We assessed metrics of the metazoan parasite infracommunities of the dusky flounder ( Syacium papillosum ) as indicators of aquatic environmental health of the Yucatan Shelf (YS) prior to oil extraction. We sampled the dusky flounder and its parasites along the YS, mostly during the 2015 north wind season (November–April). Our aims were: (i) to determine whether the parasite infracommunity metrics of S. papillosum exhibit significant differences among YS subregions; (ii) to determine whether the probability of the occurrence of its parasite species and individuals were affected by environmental variables, nutrients, heavy metals and hydrocarbons at the seascape level; and (iii) to determine whether there were statistical differences between the parasite infracommunity metrics of S. papillosum from YS and those of Syacium gunteri from the Campeche Sound. Multivariate statistical analyses and generalised additive models (GAMs) were used to examine the potential statistical associations between the contaminants, environmental variables and parasite community metrics, and the maximum entropy algorithm (MaxEnt) was used to characterise the habitat’s suitability for the parasite’s probability of occurrence. Results We recovered 48 metazoan parasite species from 127 S. papillosum , with larval cestodes and digeneans being the most numerically-dominant. Multivariate analyses showed significant differences in parasite infracommunity metrics among Western YS, Mid YS and Caribbean subregions, with the latter being the richest in species but not in individuals. The GAM and MaxEnt results indicated a negative effect of top predators (e.g. sharks and rays) removal on parasite metrics. The parasite infracommunities of S. papillosum were twice as rich in the number of species and individuals as those reported for S. gunteri from the Campeche Sound. Conclusions The significant differences among subregions in parasite metrics were apparently due to the interruption of the Yucatan current during the north wind season. The fishing of top predators in combination with an influx of nutrients and hydrocarbons in low concentrations coincides with an increase in larval cestodes and digeneans in S. papillosum . The dusky flounder inhabits a region (YS) with a larger number of metazoan parasite species compared with those available for S. gunteri in the Campeche Sound, suggesting better environmental conditions for transmission in the YS. Electronic supplementary material The online version of this article (10.1186/s13071-019-3524-6) contains supplementary material, which is available to authorized users.

Variations in discharge and turbulent kinetic energy (TKE) were studied at a point-source submarine groundwater discharge (SGD), within a fringing reef lagoon, from quadrature (neap) to syzygy (spring) tides. The principal factors affecting discharge and TKE variations were tides and waves. Field data indicated discharge, and TKE varied with high and low tides, and with quadrature and syzygy. Maximum discharge and TKE values were observed during low tides when the hydrostatic pressure over the jet was minimal, while the lowest discharge and TKE values were found at high tides. Syzygy tides produced consistent saltwater intrusion during high tides, while quadrature tides produced the greatest TKE values. In general, as the discharge intensified during low tides, jet temperatures decreased suggesting that waters originated further within the aquifer. At the same time jet salinities increased, suggesting a mixing of aquifer and seawater. To reconcile these two seemingly opposing views, it is proposed that the jet conduit is connected to a stratified chamber with seawater below brackish water. The greatest subtidal discharge occurred during quadrature tides. Syzygy produced low subtidal discharge driven by flow reversals (flow into the aquifer) observed throughout syzygy high tides in conjunction with the peak wave setup (>5 cm) observed during a storm. While tides were the primary driving force of the discharge, waves played a nonnegligible role. Wave effects on the discharge were most evident during syzygy high tides combined with a storm, when the subtidal spring discharge was weakest and salt intrusion developed.