Laura Prieto

Physical–biological coupling in the Strait of Gibraltar

This study presents a joint analysis of the distributions of some biogeochemical variables and their relation to the hydrodynamics of Gibraltar Strait. It is a synthesis paper that brings together many results obtained during CANIGO project. We show the role of hydrodynamics as a forcing agent for the plankton community structure in the Strait, with emphasis on the two physical processes that we propose as key factors for the coupling: interface position and oscillations, and mixing processes along the Strait. As a general pattern, autotrophic plankton biomass increases at the Strait from southwest to northeast, a tendency that coincides with a gradual elevation of the interface depth in the same direction. The different mechanisms of mixing that take place in the Strait are briefly reviewed: The occurrence of the internal hydraulic jump is an important mechanism of mixing constrained to the spring tide situations, but other processes such as the generation of arrested internal waves of wavelength around 1 km are proposed as a complementary mixing mechanism, particularly during neap tides situations. Both mechanisms, the elevation of the pycnocline and these mixing events, can enhance biological productivity and biomass accumulation on the northeastern sector of the Strait, since phytoplankton cells are there packaged in a water mass with sufficient light and nutrients and smaller advective velocity. There is a clear north–south difference in the biological response to these upwelling episodes in the eastern section, with high nutrient and low chlorophyll in the south and the opposite in the north. The deeper interface and the greater water speed are the proposed reasons for this lower nutrient uptake on the southeastern sector. Finally, the temporal scales of variation of the mixing events, the influence of its periodicity on the productivity of the area and the influence of these upwelling episodes in the nearest Albor! an Sea are discussed. r 2002 Elsevier Science

Environmental Control of Phase Transition and Polyp Survival of a Massive-Outbreaker Jellyfish

A number of causes have been proposed to account for the occurrence of gelatinous zooplankton (both jellyfish and ctenophore) blooms. Jellyfish species have a complex life history involving a benthic asexual phase (polyp) and a pelagic sexual phase (medusa). Strong environmental control of jellyfish life cycles is suspected, but not fully understood. This study presents a comprehensive analysis on the physicochemical conditions that control the survival and phase transition of Cotylorhiza tuberculata; a scyphozoan that generates large outbreaks in the Mediterranean Sea. Laboratory experiments indicated that the influence of temperature on strobilation and polyp survival was the critical factor controlling the capacity of this species to proliferate. Early life stages were less sensitive to other factors such as salinity variations or the competitive advantage provided by zooxanthellae in a context of coastal eutrophication. Coherently with laboratory results, the presence/absence of outbreaks of this jellyfish in a particular year seems to be driven by temperature. This is the first time the environmental forcing of the mechanism driving the life cycle of a jellyfish has been disentangled via laboratory experimentation. Projecting this understanding to a field population under climatological variability results in a pattern coherent with in situ records.

Surface distribution of chlorophyll, particles and gelbstoff in the Atlantic jet of the Alboran Sea: from submesoscale to subinertial scales of variability

The surface distribution of light attenuation due to particles (c) as well as chlorophyll-a and gelbstoff fluorescence (Fch and Fcd, respectively) were recorded during an OMEGA (EU funded, MAST III project) cruise in the northwestern Alboran Sea through a high spatial (zonally separated by 10 km and virtually meridionally continuous) and temporal (about 3 days between each of the three repeated surveys made in the zone) resolution sampling design. The distributions obtained for these variables were tightly linked to the physical forcing at the different scales that the sampling design was able to resolve. Low values dominate the quasi permanent anticyclonic gyre occupying the western Alboran Sea, whereas the frontal zone directly affected by the entrance of the Atlantic jet depicts much higher records for c, Fch and Fcd. High geostrophic Froude numbers in the jet, and the subsequent increase in turbulence diffusion of nutrients towards the surface, cannot alone justify this spatial distribution. Instead, high phytoplankton concentration at the jet could also result from the entrainment and advection of water from the upwelling zone at the Spanish coast. However, T–S characteristics suggest that this is neither the most important process for the biological enrichment of the jet, so that other mechanisms such as vertical ageostrophic velocities at the edge of the gyre must also be considered. Due to the time needed for phytoplankton growth, the intense horizontal velocities associated to the jet can decouple the sectors where deep nutrient-rich waters reach the surface from sectors where high values of the recorded variables are observed. The decoupling hinders a differentiation of this fertilization mechanism from other possible alternatives as mixing at the sills in the Strait of Gibraltar. In the third survey, the spatial structure of surface warm waters in the gyre and cold waters in the front became less apparent. ADCP data show a southward migration of the jet in a fluctuation probably related to transient states in the Atlantic jet and western Alboran gyre system. The qualitative response of c, Fch and Fcd to these scales of variability was very similar and close to the changes observed in temperature. However, the values of Fcd varied in a much narrower range than c or Fch (a factor of 2 and 10, respectively), which indicates a distinct control for the abundance of Gelbstoff. This control dumps the range of variability in the western Alboran and its origin is discussed in the context of photobleaching or bacterial degradation of these substances.

Use of a Real-Time Remote Monitoring Network (RTRM) to Characterize the Guadalquivir Estuary (Spain)

The temporal variability of hydrological variables in the Guadalquivir estuary was examined during three years through a real-time remote monitoring network (RTRM). The network was developed with the aim of studying the influence of hydrodynamical and hydrological features within the estuary on the functioning of the pelagic ecosystem. Completing this data-gathering network, monthly cruises were performed in order to measure biogeochemical variables that are indicative of the trophic status of the aquatic environment. The results showed that several sources of physical forcing, such as wind, tide-associated currents and river discharge were responsible for the spatio-temporal patterns of dissolved oxygen, salinity and turbidity in the estuary. The analysis was conducted under tidal and flood regime, which allowed us to identify river discharge as the main forcing agent of the hydrology inside the estuary. In particular, episodes of elevated turbidity detected by the network, together with episodes of low salinity and dissolved oxygen were closely related to the increase in water supply from a dam located upstream. The network installed provided accurate data that can be rapidly used for research or educational applications and by policy-makers or agencies in charge of the management of the coastal area.

Diatom aggregate formation and fluxes: a modeling analysis under different size-resolution schemes and with empirically determined aggregation kernels

A hierarchical analysis is presented for evaluating the accuracy of different formulations to simulate diatom aggregate formation and diagnose its associated carbon fluxes. We find that, for diagnostic purposes, a two-class arrangement of sizes (small particles and aggregates) is an adequate compromise between the accuracy of estimated fluxes and the level of resolution implemented for particle sizes. Ignoring the existence of aggregates severely underestimates fluxes, whereas increasing the size resolution (up to 7 size classes) yields a small improvement in accuracy. We demonstrate that this two-size-class arrangement is also the minimum resolution for accurately modeling the formation of aggregates. This arrangement requires only one aggregation kernel, which we estimate from experimental data without imposing any assumption for the interaction between particles. Although the kernel is robust to changes in the nature of the particles, the simulations obtained with this approach are very sensitive to the initial concentration of aggregates implemented to run the model. This sensitivity to initial conditions does not appear in a model with higher size resolution whose kernel tensor for aggregation has also been derived from experimental mesocosm data through an inverse modeling procedure. Although the level of accuracy achieved in a simulation with this kernel tensor seems promising, its robustness requires further tests in conditions other than a mesocosm experimental design.

Ecological aspects of early life stages of Cotylorhiza tuberculata (Scyphozoa: Rhizostomae) affecting its pelagic population success

Cotylorhiza tuberculata is a common symbiotic scyphozoan in the Mediterranean Sea. The medusae occur in extremely high abundances in enclosed coastal areas in the Mediterranean Sea. Previous laboratory experiments identified thermal control on its early life stages as the driver of medusa blooms. In the present study, new ecological aspects were tested in laboratory experiments that support the pelagic population success of this zooxanthellate jellyfish. We hypothesized that planulae larvae would have no settlement preference among substrates and that temperature would affect ephyra development, ingestion rates and daily ration. The polyp budding rate and the onset of symbiosis with zooxanthellae also were investigated. Transmission electron microscopy revealed that zooxanthella infection occurred by the polyp stage. Our results showing no substrate selectivity by planulae and high polyp budding rates in high temperatures suggest increased benthic polyp populations, which would lead to higher medusa abundances. Rates of transition from ephyrae to medusae and the feeding of early medusa stages also increased with temperature. Continuing changes in coastal ecosystems such as future climate warming and marine construction may lead to increased populations of jellyfish to the detriment of fish globally.

Portuguese Man-of-War ( Physalia physalis ) in the Mediterranean: A permanent invasion or a casual appearance?

In 2010, the Mediterranean basin experienced Portuguese Man-of-War (Physalia physalis) swarms that had dramatic consequences, including the region’s first recorded human fatality attributed to a jellyfish sting. Despite the impact of jellyfish on coastal economic activity and the importance of the tourism industry for the Mediterranean region (accounting for 15% of global tourism), no scientific consensus has been achieved regarding the causes of this episode. Here, we analyse the meteorological and oceanographic conditions of the North-East Atlantic Ocean during the months previous to the appearance of P. physalis in the Mediterranean. We simulate the probable drift of Atlantic populations into the Mediterranean basin with a numerical model and compare model results with available observations. We conclude that the summer 2010 P. Physalis swarm was the result of an unusual combination of meteorological and oceanographic conditions during the previous winter and not a permanent invasion favoured by climatic changes.

Seasonal and interannual patterns of chlorophyll bloom timing in the Gulf of Cádiz

Seasonal and interannual variability of the spring bloom in the Gulf of Cadiz, western North Atlantic, has been investigated using remote sensing chlorophyll-a (chl-a) concentration between 1997 and 2007. Variability in both the timing and magnitude of the spring bloom in the basin has been evaluated according to physical properties in the water column such as mixed layer depth (MLD), sea surface temperature (SST), sea level anomaly (SLA), absolute dynamic topography (ADT) and wind forcing. The analysis indicated that the timing, size and duration of the phytoplankton bloom in this zone is influenced by meteorological and oceanographic conditions, which means that it can vary widely from one year to another. These results in conjunction with previous studies performed in the area show that the timing of the spring bloom affects markedly the development of zooplankton, the survival of juvenile fish and the seasonality of the biological carbon pump in the Gulf of Cadiz.

First description of the life cycle of the jellyfish Rhizostoma luteum (Scyphozoa: Rhizostomeae)

Jellyfish blooms are a significant environmental problem that is increasing and may be influenced by anthropocentric practices such as overfishing, pollution, eutrophication, translocation, climate change, and ocean acidification. Many jellyfish have unknown life cycles leading to these blooms. We describe for the first time, the life cycle of scyphozoan jellyfish Rhizostoma luteum from the planula to the young medusa stages, based on laboratory observations. We also provide a preliminary assessment of temperature related to life stages. Comparisons were made with early life history stages of its sibling species Rhizostoma pulmo and Rhizostoma octopus. The life cycle of R. luteum follows the general pattern of metagenesis of scyphozoans. Scyphistoma culture was maintained in filtered seawater at 17–17.5 °C, salinity 37 and light photoperiod (12:12 h light:dark). Scyphistomae were exposed to an experimental temperature descent for two days to test their survival capacity under severe winter conditions. Only one asexual reproduction mode was observed, which is employed for propagation, consisting of podocyst formation with excystment, subsequent development of scyphistoma, strobilation and liberation of viable ephyra. The development of the ephyra to metaephyra was photodocumented, reaching the metaephyra stage in approximately 21–25 days. Young medusae grow rapidly and maturity was reached after a 3-month post-liberation period with a mean bell diameter of 13.27 ± 2.26 cm and wet weight of 181.53 ± 53 g. The life cycle of R. luteum resembles that of its congeners, with the distinction that it has the unique features of being a brooding species (internal fertilisation with subsequent release of planulae) and under the conditions tested, the predominantly strobilation type observed was monodisc, and not polydisc as with the other two species in the genus Rhizostoma. As R. luteum shows sufficient requisite to form blooms if environmental circumstances change, it is important to understand its life cycle.

The first record of Drymonema dalmatinum from the northern Alboran Sea (western Mediterranean)

The large scyphozoan jellyfish Drymonema dalmatinum was first described by Haeckel (1880) from material collected off the Dalmatian coast of the Adriatic Sea (eastern Mediterranean) . In 1882, Haeckel renamed D. dalmatinum as D. victoria, describing the species in more detail and adding a single partial sample from material collected by the HMS Challenger expedition in 1873, near Gibraltar, considering this medusa as a deep-water species. Since then, there has been no report of D. dalmatinum in the western Mediterranean. Based on genetic and morphological data, Bayha and Dawson (2010) established a new semaeostome family, Drymonematidae, to accommodate three valid Drymonema species (Bayha and Dawson 2010): the rare D. dalmatinum Haeckel, 1880 from the Mediterranean, D. larsoni Bayha and Dawson 2010 from the Caribbean, and D. gorgo Müller 1883 from the Brazilian region. Recently, Malej et al. (2014) reported an increased frequency of sightings of D. dalmatinum in the last 15 years from the Adriatic and eastern Mediterranean Sea. We report the sighting of single specimen observed on April 30, 2013, at the dive location known as Marina del Este, La Herradura, southern Spain (36.720278°N, 3.728333°W) by scuba divers (see Fig. 1). The jellyfish was found near the sea floor at a depth of 12 m, with a bell diameter of approximate 70 cm, the orientation of the umbrella was upwards, with its tentacles trailing below and accompanied by many small fishes. The sea temperature and salinity were 14.97°C and 35.082, respectively. From the photographic material taken in situ, it can be determined that the specimen belongs to D. dalmatinum showing the following morphological characters: the bell is milkish-white, shield-shaped and flatly rounded with a thicker central part (Fig. 1a). The numerous tentacles are longer than the diameter of the bell and of unequal lengths and thickness and are not grouped in separated clusters (Fig. 1b). It is to be noted that the exumbrellar markings seen in some Mediterranean animals are not visible in this individual, but may be imperceptible because of the quality of the photographic material (Fig. 1c). Figure 1d shows the complexly folded veil-like oral arms, which are almost as long as the bell diameter. The genus Drymonema is distinguished from Cyanea and Desmonema by the fact that its tentacles arise from the middle zone of the subumbrella and not in separated clusters. Haeckel’s finding from the Strait of Gibraltar has been questioned by Mayer (1910) and being such a rare medusa, i t seemed important to document this occurrence. Communicated by S. Piraino