Modelling the relationship between sexual reproduction and rhizome growth in Posidonia oceanica (L.) Delile

The relationship between flowering and growth performance of Posidonia oceanica (L.) Delile in meadows distributed along the south‐eastern coast of Sicily (Italy) was investigated by means of a statistical model (generalized linear mixed model) combined with the lepidochronological analysis. Over a 28‐year period, 67 floral stalk remains were observed. The highest flowering index was recorded in lepidochronological year 1998 (10.1%) and the Inflorescence Frequency per age showed a clear decrease corresponding to 15‐year‐old shoots. The sexual reproductive event had positive effects on rhizome elongation (cm year^?1) and leaf production (no. leaves year^?1) in the same flowering year, whilst no effect on the rhizome production (mg year^?1) was observed. Rhizome growth variables showed significant negative lagged responses in the two years following flowering. On the whole, we calculated that the effect exerted by flowering, in terms of loss on rhizome elongation and production, was about 27% and 38%, respectively. Although it has been demonstrated that recovery from the stress induced by sexual reproduction is limited to the two years after flowering, the magnitude of the reproductive cost may become quite considerable especially in comparison with the whole lifespan of individual shoots.     

Southern elephant seal trajectories, fronts and eddies in the Brazil/Malvinas Confluence

This study describes the association between transient, mesoscale hydrographic features along the axis of the Brazil–Malvinas Confluence, in the SW Atlantic, and the foraging behavior of 2–3-year-old (focal) juvenile southern elephant seals, Mirounga leonina, from Península Valdés, Argentina. Departing from the dominant pattern of foraging on predictable bathymetric fronts on the Patagonian shelf and slope, three females out of 12 satellite-tracked juveniles remained at the edge of young warm-core eddies and near the outer core of cold-core eddies, coinciding with the most productive areas of these temperature fronts. Seal trajectories along high-temperature gradients were always consistent with the speed and direction of surface currents inferred from the temperature distribution and confirmed by surface drifters. Movements of foraging seals were compared with those of surface drifters, coinciding in time and space and yielding independent and consistent data on regional water circulation parameters. The diving pattern recorded for one focal seal yielded shallower dives and a loose diel pattern in the eddy, and a marked diurnal cycle compatible with foraging on vertically migrating prey in the cold waters of the Malvinas Current. Pre-reproductive females that use the mesoscale fronts of the Argentine Basin as an alternative foraging area would benefit from lower competition with more experienced seals and with other top predators that reproduce along the coast of Patagonia.     

Spatio-Temporal Distribution of Algal and Animal Communities in a Posidonia oceanica Meadow

Abstract. Investigations were carried out in a Posidonia oceanica meadow at Ischia (Gulf of Naples) along a depth gradient from 1 to 32 m in November 1979 and May 1981.
In these two months, different discontinuities were found at the sampling stations (1, 5, 10, 20, 30 m) at 10 m for leaf features, between 1 and 5 m for the algal community and the vagile fauna. The leaf features (length, surface, and biomass) along the transects show a delay in leaf production toward the deeper stations, below the thermocline.
The algal community shows in both seasons and in all stations a persistence of an encrusting layer, mostly represented by Corallinaceae and the brown alga Myrionema orbiculare , while at the shallowest station (1 m) the community is characterized by a more developed upright layer.
The vagile fauna of the leaf stratum, mostly represented by Polychaeta, Mollusca , and Crustacea (Peracarida and Decapoda) , shows a consistent zonation in both seasons along the transect. A superficial community at I m, characterized by a low number of species and individuals and specialized for high environmental energy levels, and a deeper community, persistent in time and more strictly related to the Posidonia oceanica meadows, are identified.
The influence of environmental factors and the importance of meadow structure for the zonation of the algal and animal communities are discussed.     

Temporal and spatial variations in the biogeochemical cycling of cobalt in two urban estuaries: Hudson River Estuary and San Francisco Bay

Despite the fact that Co is an essential trace element for the growth of marine phytoplankton, there is very limited information on the cycling of this trace metal in the marine environment. We report here the distribution of dissolved (<0.4 μm) and particulate (>0.4 μm) Co in surface waters of the Hudson River Estuary (HRE) and San Francisco Bay (SFB). Samples were collected during several cruises (from 1990 to 1995 in SFB and from 1995 to 1997 in the HRE) along the whole salinity gradient. Dissolved Co concentrations (mean±1 standard deviation) were nearly identical in magnitude in both estuaries despite differences in climate, hydrography, riverine-flow conditions and land-usage (HRE=0.91±0.61 nM; SFB=1.12±0.69 nM). Dissolved Co levels in each system showed non-conservative distributions when plotted as a function of salinity, with increasing concentrations downstream from the riverine end-members. Desorption from suspended particulates and sewage inputs, therefore, seems to be the major processes responsible for the non-conservative behavior of Co observed. Mass balance estimates also indicated that most of the estuarine Co is exported out of both estuaries, indicating that they and other estuarine systems are principal sources of this essential trace element to the open ocean.     

In Situ Measurement of Pinna nobilis Shells for Age and Growth Studies: A New Device

Abstract. Pinna nobilis Linnaeus 1758 is an endemic bivalve mollusc in the Mediterranean Sea, where it inhabits seagrass meadows, especially Posidonia oceanica (L.) Delile. It is the largest bivalve in the Mediterranean, reaching lengths up to 120 cm. In its natural habitat, P. nobilis lives with the anterior part of the valve buried in the seabed, attached to Posidonia rhizomes by byssus threads.
This habit makes it impossible to measure its total length directly in situ. As the only way to determine the individual age is the relationship between age and total length, several equations have been proposed to estimate total length by relating it to the unburied parts of the shell. Such measurements are essential to ecological studies that consider age, growth, and population dynamics, and that evaluate the environmental factors that affect this species.
Accurately estimating total length depends on the accuracy and precision of the method employed to measure the unburied shell parts. In this paper, we point out the lack of precision of the instruments and methods used until now; we also demonstrate the reason for this imprecision. A new device to measure unburied parts of Pinna nobilis with a precision comparable to that obtained when measuring extracted valves is described. This device is unaffected by substratum type and reduces measurement time. The latter is a very important feature, because these procedures are usually performed whilst SCUBA diving. Finally, a growth equation has been fitted to the measurements obtained with the new device from a population located in Moraira (Alicante, western Mediterranean).     

Optimizing interpolation of shoot density data from a Posidonia oceanica seagrass bed

A case study on the optimization of Posidonia oceanica density interpolation, using a data set from a large meadow at Porto Conte Bay (NW Sardinia, Italy), is presented. Ordinary point kriging, cokriging and a weighted average based on inverse square distance were used to interpolate density data measured in 36 sampling stations. The results obtained from different methods were then compared by means of a leave‐one‐out cross‐validation procedure. The scale at which interpolation was carried out was defined on the basis of the Hausdorff dimension of the variogram. Optimizing spatial scale and data points search strategy allowed obtaining more accurate density estimates independently of the interpolation method.     

Mid-Holocene emergence of southern Tunisian coasts

We present new sea-level data from the coasts of southern Tunisia, between the Gulf of Gabès and the Libyan border. The work tests, previously, published evidence on Holocene shorelines, and confirmed that a distinct emergence has occurred in this area during this time. The emergence peak lies at least 186 ± 11 cm above present and is inferred from: (1) AMS radiocarbon dates of subtidal vermetids and boring shells collected in growth position, and (2) careful assessment of tidal heights. Maximum emergence took place between about 6000 and 5000 ¹⁴C years BP; it cannot be ascribed to tectonics and is probably related to post-glacial hydro-isostatic effects. It challenges the inference of a 3-m global sea-level rise since 6000 years BP due to residual Antarctic melting.