Spatial patterns and environmental risks of ringnet fishing along the Kenyan coast

Ringnet fishing began in the early 20th century and is practised worldwide, mainly to target nearshore pelagic species. The method was introduced to Kenya’s coastal waters by migrant fishers from Tanzania. However, the impacts of this fishing gear remain poorly assessed. We assessed the spatial distribution of ringnet fishing effort and its possible effects on ecosystem components, such as coral reefs, marine megafauna and marine protected areas, on the south coast of Kenya. We tracked 89 ringnet fishing trips made from December 2015 to January 2016 and used spatial multicriteria analysis to determine hotspots of possible environmental risks. The results showed that habitat type and bathymetric profile influenced the spatial distribution of ringnet fishing effort. Mixed seagrass and coral habitats had the highest concentration of the effort. Most of the habitats in the study area were moderately exposed to the impacts of the ringnet fishery. The study identifies high-risk areas that require spatial measures to minimise possible environmental risks of the gear both to habitats and to endangered sea turtles.     

Management of a shallow temperate estuary to control eutrophication: The effect of hydrodynamics on the system’s nutrient loading

The Mondego estuary, a shallow warm-temperate intertidal system located on the west coast of Portugal, has for some decades been under severe ecological stress, mainly caused by eutrophication. Water circulation in this system was, until 1998, mainly dependent on tides and on the freshwater input of a small tributary artificially controlled by a sluice. After 1998, the sluice opening was effectively minimised to reduce the nutrient loading, and the system hydrodynamics improved due to engineering work in the upstream areas. The objective of the present study was to evaluate the effect of the mitigation measures implemented in 1998. Changes to the hydrodynamics of the system were assessed using precipitation and salinity data in relation to the concentrations of dissolved inorganic nutrients, as well as the linkage between dissolved N:P ratios and the biological parameters (phytoplankton chlorophyll a concentrations, green macroalgal biomass and seagrass biomass). Two distinctive periods were compared, over a ten year period: from January 1993 to January 1997 and from January 1999 until January 2003. The effective reduction in the dissolved N:P atomic ratio from 37.7 to 13.2 after 1998 is a result of lowered ammonia, but not the oxidised forms of nitrogen (nitrate plus nitrite), or increased concentrations of dissolved inorganic phosphorus. Results suggest that the phytoplankton is not nutrient limited, yet maximum and mean biomass of green macroalgae was reduced by one order of magnitude after the mitigation measures. This suggests that besides lowering the water residence time of the system, macroalgal growth became nitrogen limited. In parallel to these changes the seagrass-covered area and biomass of Zostera noltii showed signs of recovery.     

Calibration of JASON-1 Altimeter over Lake Erie

This article describes absolute calibration results for both JASON-1 and TOPEX Side B (TSB) altimeters obtained at the Lake Erie calibration site, Marblehead, Ohio, USA. Using 15 overflights, the estimated JASON altimeter bias at Marblehead is 58 ± 38 mm, with an uncertainty of 19 mm based on detailed error analysis. Assuming that the TSB bias is negligible, relative bias estimates using both data from the TSB-JASON formation flight period and data from 48 water level gauges around the entire Great Lakes confirmed the Marblehead results. Global analyses using both the formation flight data and dual-satellite (TSB and JASON) crossovers yield a similar relative bias estimate of 146 ± 59 mm, which agrees well with open ocean absolute calibration results obtained at Harvest, Corsica, and Bass Strait (e.g., Watson et al. 2003). We find that there is a strong dependence of bias estimates on the choice of sea state bias (SSB) models. Results indicate that the invariant JASON instrument bias estimated oceanwide is 71 mm, with additional biases of 76 mm or 28 mm contributed by the choice of Collecte Localisation Satellites (CLS) SSB or Center for Space Research (CSR) SSB model, respectively. Similar analysis in the Great Lakes yields the invariant JASON instrument bias at 19 mm, with the SSB contributed biases at 58 mm or 13 mm, respectively. The reason for the discrepancy is currently unknown and warrants further investigation. Finally, comparison of the TOPEX/POSEIDON mission (1992-2002) data with the Great Lakes water level gauge measurements yields a negligible TOPEX altimeter drift of 0.1 mm/yr.     

Effects of the water-soluble fraction of microbiologically or physically altered crude petroleum on the heterotrophic activity of marine bacteria

Crude petroleum was emulsified or degraded by marine microorganisms and physically altered by an artificial weathering process. Only the water-soluble fraction from emulsified crude petroleum inhibited glutamate uptake by marine microorganisms from South Carolina and Georgia coastal waters. Chloroform extraction recovered almost equal amounts of total hydrocarbon from each water-soluble fraction. Chromatographic data indicated that the composition of microbially emulsified petroleum was similar to that of microbially degraded petroleum. Emulsification of crude petroleum was only observed in summer water samples and did not consistently occur at a given location. Results of this study indicate that the process by which crude petroleum is altered will influence its toxicity to microbial heterotrophic activity.     

Submerged cylinder wave energy device: theory and experiment

Linearized water wave theory is used to show that a submerged long circular cylinder suitably constrained by springs and dampers to make small harmonic oscillations, can be extremely efficient in absorbing the energy in an incident regular wave whose crests are parallel to the axis of the cylinder. Experimental results are described which confirm the theory for small amplitude waves and which suggest that the device can still be fairly efficient in waves of moderate amplitude.     

Solution of an unstable axisymmetric Cauchy–Poisson problem of dispersive water waves for a spectrum with compact support

 It has been known that the axisymmetric Cauchy–Poisson problem for dispersive water waves is well posed in the sense of stability. Thereby time evolution solutions of wave propagation depend continuously on initial conditions. However, in this paper, it is demonstrated that the axisymmetric Cauchy–Poisson problem is ill posed in the sense of stability for a certain class of initial conditions, so that the propagating solutions do not depend continuously on the initial conditions. In order to overcome the difficulty of the discontinuity, Landweber–Fridman's regularization, famous and well known in applied mathematics, are introduced and investigated to learn whether it is applicable to the present axisymmetric wave propagation problem. From the numerical experiments, it is shown that stable and accurate solutions are realized by the regularization, so that it can be applicable to the determination of the ill-posed Cauchy–Poisson problem.     

Organic matter origin and distribution in suspended particulate materials and surficial sediments from the western Adriatic Sea (Italy)

In this study, organic carbon (OC), total nitrogen (TN), stable carbon isotopic (δ¹³C_OC) and CuO reaction product compositions were used to identify the sources of organic matter (OM) and to quantify the relative importance of allochthonous and autochthonous contributions to the western Adriatic Sea, Italy. Suspended particulate material (195 samples) and surficial sediments (0–1 cm, 70 samples) from shallow cross-shelf transects were collected in February and May 2003, respectively. Vertical water column profiles were acquired along the same transects. Data include depth, potential temperature, salinity, density and chlorophyll fluorimetry.Along the western Adriatic shelf in the near-shore region, the phytoplankton growth was influenced by dynamics of the buoyant plumes from the Po and Appennine rivers. A small amount of very fine terrigenous material remained suspended within the coastal current and was exported southward along the shelf to the slope. High variability in the bulk composition was detected in the Po prodelta surficial sediments, whereas the western Adriatic shelf, although a larger area, exhibited a narrower range of values.A significant decoupling was observed between suspended particles in the water column and surficial deposits. The organic material collected in the water column was compositionally heterogeneous, with contributions from marine phytoplankton, riverine–estuarine phytoplankton and soil-derived OM. Frequent physical reworking of surficial sediments likely leads to the efficient oxidation of marine OC, resulting in the observed accumulation and preservation of refractory soil-derived OC delivered by the Po and Appennine rivers.     

First results from the North Iceland experiment

Between June 2004 and September 2004 a temporary seismic network was installed on the northern insular shelf of Iceland and onshore in north Iceland. The seismic setup aimed at resolving the subsurface structure and, thus, the geodynamical transition from Icelandic crust to typical oceanic crust along Kolbeinsey Ridge. The experiment recorded about 1,000 earthquakes. The region encloses the Tjörnes Fracture Zone containing the Husavik–Flatey strike-slip fault and the extensional seismic Grimsey Lineament. Most of the seismicity occurs in swarms offshore. Preliminary results reveal typical mid-ocean crust north of Grimsey and a heterogeneous structure with major velocity anomalies along the seismic lineaments and north–south trending subsurface features. Complementary bathymetric mapping highlight numerous extrusion features along the Grimsey Lineament and Kolbeinsey Ridge. The seismic dataset promises to deliver new insights into the tectonic framework for earthquakes in an extensional transform zone along the global mid-ocean ridge system.