Flow disruption resulting from interactions between currents and abrupt topography can have important consequences for biological processes in the ocean. A highresolution three-dimensional hydrodynamic model is used to study topographically influenced flow at the Flower Garden Banks, two small but thriving coral reef ecosystems in the northwest Gulf of Mexico. Flow past the modeled banks is characterized by vortex shedding, turbulent wake formation and strong return velocities in the near-wake regions. The speed of the oncoming current, strength of water-column stratification, and level of topographic detail used in the model each serve to modulate these basic flow characteristics. Larval retention and dispersal processes at the Flower Garden Banks, and specifically the dependence of these processes on the nature of flow disruption, are explored by coupling a Lagrangian particle-tracking algorithm to the hydrodynamic model. Passive particles released from the tops of the modeled banks as mimics of coral larvae can remain trapped in the wake regions very close to the banks on time scales of hours to days, depending primarily on the speed of the free-stream current. Most particles are swept quickly downstream, however, where their trajectories are most strongly influenced by the topography of the continental shelf. Modeled dispersal patterns suggest that there is an ample supply of larvae from the Flower Garden Banks to nearby oil and gas platforms, which can provide suitable benthic habitat for corals. The flow disturbances generated by the modeled banks result in the mixing of nutrients from deeper water into shallower, nutrient-depleted layers in the wakes of the banks. The ability of the planktonic system to respond to such an injection of nutrients is tested by embedding a simple nutrient-phytoplankton-zooplankton ecosystem model into the hydrodynamic model. Plankton biomass in the flow-disturbed wakes is shown to increase in response to the additional nutrients. This study shows how flow-topography interactions at the Flower Garden Banks can exert critical control over local larval transport processes and plankton dynamics. More generally, it demonstrates the usefulness and feasibility of using numerical models as tools to uncover important mechanisms of physical-biological interaction in the ocean.

The supply of coral reef fish larvae from the open ocean to reefs is vital for the persistence of local fish populations. Whether larvae are dispersed over hundreds of km or only few km depends on biophysical interactions between larvae and their environment. Relationships between environmental variables, larval swimming behavior, and larval transport were examined for reef fish larvae in the Florida Straits. In a series of research cruises, the upper 100 m of the water column was sampled with plankton nets fishing at four different depths. Variability in the vertical distributions of most larvae was not consistently related to measured environmental variables. Relative densities of larvae were predictably related to sampling depth in five taxa. In seven taxa, more developed larvae were distributed significantly deeper than less developed larvae, revealing ontogenic vertical migrations. In three taxa, vertical distributions varied significantly between day and night, revealing diel migrations. Since the Florida Current was strongest near the surface, observed vertical distributions and migrations resulted in reduced larval transport relative to surface currents. To identify cues involved in regulating vertical distributions, behavioral experiments were conducted with larvae from four reef fish families. All four groups showed significant responses to pressure cues, swimming up in response to high pressure and down in response to low pressure. In two families there was a significant correlation between capture depth and experimental pressure preference, suggesting that larvae use similar behavior to regulate depth in situ. To study horizontal swimming behavior, late-stage larvae of one species were caught in light-traps and observed by SCUBA divers ~1 km offshore of the Florida Keys barrier reef. All larvae swam remarkably straight, but their swimming directions were distributed randomly. A simulation model was used to generate swimming trajectories of longer duration than could be observed directly. Observed and simulated trajectories indicated that horizontal swimming by larvae with or without an external reference frame was important at spatial scales of several km. Overall, some larvae exercised a strong influence on transport, either by vertical or horizontal swimming. Behaviors varied between species and families, highlighting the need for more species-specific data.

O presente estudo foi realizado ao longo de uma linha de costa, incluindo duas enseadas em Ubatuba-SP, Brasil. Os objetivos principais foram: (1) Examinar, em seis costões rochosos, séries temporais de assentamento larval de decápodos, afim de entender forçantes físicas associadas ao transporte em direção à costa, (2) Localizar agregações larvais no plâncton neustônico costeiro, para inferir a natureza das agregações e verificar a ocorrência de migração vertical em decápodos, (3) testar a relação entre suprimento e assentamento larval, a diferentes escalas espaciais. O papel da competência larval foi discutido à luz de evidências obtidas através amostragem planctônica em diferentes áreas dentro das enseadas e comparações inter e intra-taxonômicas. Entre maio e agosto de 2005, foram realizadas coletas, a cada dois dias, para estimar a taxa de assentamento larval de grupos de decápodes em coletores artificiais. Durante dois períodos de maior assentamento de Pachygrapsus transversus, a ocorrência nictemeral neustônica, desta e de outras espécies de decápodes foi avaliada. Sub-amostras de megalopas de brachyura foram separadas para mensuração do nível de competência larval, através do cultivo de larvas, obtendo estimativas do tempo decorrido até a muda, e através do estágio de muda de indivíduos previamente fixados. Modelos Log-lineares foram utilizados para testar o efeito do vento e da maré na taxa de assentamento, e os resultados foram confrontados com cenários de correntes superficiais fornecidos pelo modelo hidrodinâmico de circulação. Amostras planctônicas revelaram que agregações larvais são coincidentes com zonas de convergência, provavelmente causadas pela formação de ondas internas. Um conjunto de espécies de camarão sofre migração vertical típica, enquanto um padrão de migração reversa foi encontrado para grapsídeos, ix o que pode auxiliar no transporte em direção à costa para as áreas de assentamento devido a brisas marinhas. Portunídeos não apresentaram tendência de migração vertical. A competência parece ser melhor estimada através do método de cultivo. Em áreas dentro das enseadas, megalopas de portunídeos parecem ser mais competentes ao assentamento que as de grapsídeos, provavelmente por estarem mais próximas aos locais de assentamento. Megalopas neustônicas de P. transversus coletadas dentro das enseadas são menos competentes que as obtidas em coletores artificiais. O estágio transicional não foi capturado, sugerindo que a transição para o bentos e o desenvolvimento até estágios competentes são mais complexos do que normalmente assumido. Não foi encontrada correlação significativa entre o suprimento de megalopas e a taxa de assentamento. A taxa de assentamento de decápodes, que habitam a região entre-marés de costões rochosos, é aparentemente dependente da interação das forçantes de vento, maré e comportamento larval. As taxas de assentamento de grapsídeos e palemonídeos seguem o ritmo semi-lunar, com ocorrência máxima próximo às marés de quadratura, corroborando a hipótese de que ondas internas geradas pela maré são também responsáveis pelo transporte de decápodos costeiros. Ventos em direção à costa foram identificados como agentes chave, apesar de dependentes do regime de marés, enquanto que ventos de nordeste provavelmente trazem larvas de locais de produção. Padrões de circulação costeira e contrastes de assentamento consistentes espacialmente indicam que a Enseada do Flamengo é uma importante área de deposição de larvas. / The present study was carried out along a coastline including two bays in Ubatuba, SP, Brazil. The main objectives were (1) examine, at six different rocky shores, the time series of decapod larval settlement in order to understand the physical forcing associated to onshore transport, (2) locate neustonic plankton larval aggregations over the diel cycle to infer on the nature of patchiness and to verify the occurrence of vertical migration in the decapod species examined, and (3) test the relationship between larval supply and settlement, at different spatial scales. The role of larval competency was discussed based on evidence obtained by sampling the plankton at distinct within-bay areas and comparing the results both within and between taxa. From May to August 2005, samples were carried out every other day to estimate the larval settlement rate of two different decapod assemblages in artificial collectors. During two settlement pulses recorded for the megalopae of Pachygrapsus transversus, the nycthemeral neustonic occurrence of that and other decapod stages was assessed. Subsamples of brachyuran megalopae were separated to measure their competency level, by means of both larval rearing, thus obtaining time-to-moult estimates, and microscopic moult-staging of readily fixed individuals. Log-linear models were used to test the effect of wind and tide range on settlement rate, and results were confronted to possible scenarios of surface currents provided by hydrodynamic numerical modeling. Planktonic samples revealed that patches of larvae are coincident to slick fringes, probably caused by the formation of internal waves. A suite of shrimp species undertakes a typical diel vertical migration, while a reversed pattern was found for grapsids, which may assist the final onshore transport to settlement grounds due to marine breezes. Swimming crabs did not show any vertical xi migration trend. Competency is suggested to be better estimated using the larval rearing method. In within-bay areas, portunid megalopae appeared to be more competent to settle than those of grapsids, probably because the former are closer to settlement substrates than the latter, which should be first advected to the nearshore. Within-taxon comparisons revealed that neustonic megalopae of P. transversus collected in within-bay areas are less competent than those obtained from benthic collectors. A transitional stage was missing, suggesting that the transition to the benthos and the development into competent stages are more complex than usually assumed. Significant correlations between megalopae supply and settlement rate were not found. The settlement rate of decapods inhabiting the intertidal region of the rocky coast sampled is apparently dependent on the interaction of wind and tide forcing coupled with larval behaviour. Settlement rate of both grapsids and palaemonids follows a semilunar rhythm, with maxima occurring near neap tides, corroborating the hypothesis that tidally-driven internal motions are also responsible for onshore larval transport coastal decapods. Onshore wind-forcing was found to be the key agent, although dependent on the tide regime, while winds blowing from northeast are suggested to bring larvae from source sites. Coastal circulation patterns and spatially-consistent settlement contrasts indicate that Flamengo Bay is an important area of larval deposition.

O presente estudo foi realizado ao longo de uma linha de costa, incluindo duas enseadas em Ubatuba-SP, Brasil. Os objetivos principais foram: (1) Examinar, em seis costões rochosos, séries temporais de assentamento larval de decápodos, afim de entender forçantes físicas associadas ao transporte em direção à costa, (2) Localizar agregações larvais no plâncton neustônico costeiro, para inferir a natureza das agregações e verificar a ocorrência de migração vertical em decápodos, (3) testar a relação entre suprimento e assentamento larval, a diferentes escalas espaciais. O papel da competência larval foi discutido à luz de evidências obtidas através amostragem planctônica em diferentes áreas dentro das enseadas e comparações inter e intra-taxonômicas. Entre maio e agosto de 2005, foram realizadas coletas, a cada dois dias, para estimar a taxa de assentamento larval de grupos de decápodes em coletores artificiais. Durante dois períodos de maior assentamento de Pachygrapsus transversus, a ocorrência nictemeral neustônica, desta e de outras espécies de decápodes foi avaliada. Sub-amostras de megalopas de brachyura foram separadas para mensuração do nível de competência larval, através do cultivo de larvas, obtendo estimativas do tempo decorrido até a muda, e através do estágio de muda de indivíduos previamente fixados. Modelos Log-lineares foram utilizados para testar o efeito do vento e da maré na taxa de assentamento, e os resultados foram confrontados com cenários de correntes superficiais fornecidos pelo modelo hidrodinâmico de circulação. Amostras planctônicas revelaram que agregações larvais são coincidentes com zonas de convergência, provavelmente causadas pela formação de ondas internas. Um conjunto de espécies de camarão sofre migração vertical típica, enquanto um padrão de migração reversa foi encontrado para grapsídeos, ix o que pode auxiliar no transporte em direção à costa para as áreas de assentamento devido a brisas marinhas. Portunídeos não apresentaram tendência de migração vertical. A competência parece ser melhor estimada através do método de cultivo. Em áreas dentro das enseadas, megalopas de portunídeos parecem ser mais competentes ao assentamento que as de grapsídeos, provavelmente por estarem mais próximas aos locais de assentamento. Megalopas neustônicas de P. transversus coletadas dentro das enseadas são menos competentes que as obtidas em coletores artificiais. O estágio transicional não foi capturado, sugerindo que a transição para o bentos e o desenvolvimento até estágios competentes são mais complexos do que normalmente assumido. Não foi encontrada correlação significativa entre o suprimento de megalopas e a taxa de assentamento. A taxa de assentamento de decápodes, que habitam a região entre-marés de costões rochosos, é aparentemente dependente da interação das forçantes de vento, maré e comportamento larval. As taxas de assentamento de grapsídeos e palemonídeos seguem o ritmo semi-lunar, com ocorrência máxima próximo às marés de quadratura, corroborando a hipótese de que ondas internas geradas pela maré são também responsáveis pelo transporte de decápodos costeiros. Ventos em direção à costa foram identificados como agentes chave, apesar de dependentes do regime de marés, enquanto que ventos de nordeste provavelmente trazem larvas de locais de produção. Padrões de circulação costeira e contrastes de assentamento consistentes espacialmente indicam que a Enseada do Flamengo é uma importante área de deposição de larvas. / The present study was carried out along a coastline including two bays in Ubatuba, SP, Brazil. The main objectives were (1) examine, at six different rocky shores, the time series of decapod larval settlement in order to understand the physical forcing associated to onshore transport, (2) locate neustonic plankton larval aggregations over the diel cycle to infer on the nature of patchiness and to verify the occurrence of vertical migration in the decapod species examined, and (3) test the relationship between larval supply and settlement, at different spatial scales. The role of larval competency was discussed based on evidence obtained by sampling the plankton at distinct within-bay areas and comparing the results both within and between taxa. From May to August 2005, samples were carried out every other day to estimate the larval settlement rate of two different decapod assemblages in artificial collectors. During two settlement pulses recorded for the megalopae of Pachygrapsus transversus, the nycthemeral neustonic occurrence of that and other decapod stages was assessed. Subsamples of brachyuran megalopae were separated to measure their competency level, by means of both larval rearing, thus obtaining time-to-moult estimates, and microscopic moult-staging of readily fixed individuals. Log-linear models were used to test the effect of wind and tide range on settlement rate, and results were confronted to possible scenarios of surface currents provided by hydrodynamic numerical modeling. Planktonic samples revealed that patches of larvae are coincident to slick fringes, probably caused by the formation of internal waves. A suite of shrimp species undertakes a typical diel vertical migration, while a reversed pattern was found for grapsids, which may assist the final onshore transport to settlement grounds due to marine breezes. Swimming crabs did not show any vertical xi migration trend. Competency is suggested to be better estimated using the larval rearing method. In within-bay areas, portunid megalopae appeared to be more competent to settle than those of grapsids, probably because the former are closer to settlement substrates than the latter, which should be first advected to the nearshore. Within-taxon comparisons revealed that neustonic megalopae of P. transversus collected in within-bay areas are less competent than those obtained from benthic collectors. A transitional stage was missing, suggesting that the transition to the benthos and the development into competent stages are more complex than usually assumed. Significant correlations between megalopae supply and settlement rate were not found. The settlement rate of decapods inhabiting the intertidal region of the rocky coast sampled is apparently dependent on the interaction of wind and tide forcing coupled with larval behaviour. Settlement rate of both grapsids and palaemonids follows a semilunar rhythm, with maxima occurring near neap tides, corroborating the hypothesis that tidally-driven internal motions are also responsible for onshore larval transport coastal decapods. Onshore wind-forcing was found to be the key agent, although dependent on the tide regime, while winds blowing from northeast are suggested to bring larvae from source sites. Coastal circulation patterns and spatially-consistent settlement contrasts indicate that Flamengo Bay is an important area of larval deposition.

Nos estudos de populações pesqueiras é comum o uso de modelos hidrodinâmicos acoplados a modelos biológicos e, um tipo de modelo merece destaque: o baseado no indivíduo (IBM). A Plataforma Continental do Atlântico Sudoeste comporta uma importante atividade pesqueira e, a influência da circulação na distribuição de ovos/larvas nessa região é pouco entendida. Estudos anteriores sugerem que o transporte de Ekman em direção à costa e o deságue de água fluvial impedem a perda de ovos e larvas da área costeira. Este trabalho apresenta resultados de um estudo englobando os padrões de circulação e de transporte de larvas nessa área, realizado através do desenvolvimento de um IBM para os estágios iniciais da Engraulis anchoita e o acoplamento deste a dois campos de velocidade superficial: a de Ekman, e a saída de um modelo hidrodinâmico 3D. Os resultados mostram que somente a deriva de Ekman não é suficiente para explicar o padrão de retenção larval observado na região. Os valores de retenção encontrados com a utilização da saída do modelo estão próximos aos observados. Maiores concentrações larvais foram obtidas no outono/inverno e menores na primavera/verão. Estes resultados estão diretamente relacionados à combinação dos efeitos dos padrões de vento da área com a influência das Correntes do Brasil e das Malvinas e do deságue continental na circulação sobre a plataforma. / Recruitment of pelagic fishes is the net result a series of continuous events in their life cycle. Most of the variability in natural mortality is thought to occur during the early stages of development and is related to environmental factors. The use of coupled physical-biological modeling in the study of fisheries populations is very common and a particular type of model deserves special mention: the Individual Based Model (IBM). The Atlantic Southwestern Continental Shelf is one of the most productive fishing sites in the Western South Atlantic, but the processes involving the circulation in this area and its influence on eggs/larvae transport and retention are still poorly understood. Recent studies describe the spawning activity of anchovy and its relationship to oceanographic processes and suggest that predominant onshore Ekman transport, during the austral winter/spring, keeps the eggs and larvae in coastal areas. According to these studies, the river runoff contributes to the formation of a retention zone for planktonic organisms over the area. This work presents results of a study that considers both the circulation pattern and larval transport, throught the development of an IBM for the early stages of anchovy Engraulis anchoita. The model is coupled to two surface velocity fields: the Ekman drift, and the output of a 3D hydrographic model. The results show that just Ekman surface velocities are not able to explain the larval dispersion observed in this region. Retention values obtained with the model output are much closer to observed values. Autumn and winter seasons present the highest values of retention. These results are due to the combined effect of the wind patterns registered in the area with the influence of the river runoff and the Brazil-Malvinas shelf circulation.

Nos estudos de populações pesqueiras é comum o uso de modelos hidrodinâmicos acoplados a modelos biológicos e, um tipo de modelo merece destaque: o baseado no indivíduo (IBM). A Plataforma Continental do Atlântico Sudoeste comporta uma importante atividade pesqueira e, a influência da circulação na distribuição de ovos/larvas nessa região é pouco entendida. Estudos anteriores sugerem que o transporte de Ekman em direção à costa e o deságue de água fluvial impedem a perda de ovos e larvas da área costeira. Este trabalho apresenta resultados de um estudo englobando os padrões de circulação e de transporte de larvas nessa área, realizado através do desenvolvimento de um IBM para os estágios iniciais da Engraulis anchoita e o acoplamento deste a dois campos de velocidade superficial: a de Ekman, e a saída de um modelo hidrodinâmico 3D. Os resultados mostram que somente a deriva de Ekman não é suficiente para explicar o padrão de retenção larval observado na região. Os valores de retenção encontrados com a utilização da saída do modelo estão próximos aos observados. Maiores concentrações larvais foram obtidas no outono/inverno e menores na primavera/verão. Estes resultados estão diretamente relacionados à combinação dos efeitos dos padrões de vento da área com a influência das Correntes do Brasil e das Malvinas e do deságue continental na circulação sobre a plataforma. / Recruitment of pelagic fishes is the net result a series of continuous events in their life cycle. Most of the variability in natural mortality is thought to occur during the early stages of development and is related to environmental factors. The use of coupled physical-biological modeling in the study of fisheries populations is very common and a particular type of model deserves special mention: the Individual Based Model (IBM). The Atlantic Southwestern Continental Shelf is one of the most productive fishing sites in the Western South Atlantic, but the processes involving the circulation in this area and its influence on eggs/larvae transport and retention are still poorly understood. Recent studies describe the spawning activity of anchovy and its relationship to oceanographic processes and suggest that predominant onshore Ekman transport, during the austral winter/spring, keeps the eggs and larvae in coastal areas. According to these studies, the river runoff contributes to the formation of a retention zone for planktonic organisms over the area. This work presents results of a study that considers both the circulation pattern and larval transport, throught the development of an IBM for the early stages of anchovy Engraulis anchoita. The model is coupled to two surface velocity fields: the Ekman drift, and the output of a 3D hydrographic model. The results show that just Ekman surface velocities are not able to explain the larval dispersion observed in this region. Retention values obtained with the model output are much closer to observed values. Autumn and winter seasons present the highest values of retention. These results are due to the combined effect of the wind patterns registered in the area with the influence of the river runoff and the Brazil-Malvinas shelf circulation.