Spatial variation of nutrients and primary productivity in the Rufiji Delta mangroves,Tanzania

Determinations of spatial and temporal variations in organic matter and nutrient dynamics in water and sediments are crucial for understanding changes in aquatic bodies. In this study, we (i) determine the spatial dynamics of dissolved inorganic nutrients, during the transition from the dry to the rainy season, and (ii) provide future productivity predictions for the Rufiji Delta mangroves, Tanzania, based on the input of various nutrients. Water samples were collected from six locations, three times per year between April 2012 and January 2014, and analysed for dissolved nutrients, total organic and inorganic carbon, chlorophyll a, chlorophyll b and total carotenoids. The prediction of future net primary productivity in the Rufiji mangroves was undertaken using the software STELLA. The mean nutrient concentrations were of the order: nitrate > phosphate > ammonium > silica > dissolved organic carbon. The study revealed that high nutrient concentrations occurred in the northern part of the Rufiji Delta as a result of anthropogenic influence in the watershed. Modelling of nutrient inputs into the delta indicated enhanced primary productivity, which is expected to increase the vulnerability of water quality in the near future due to eutrophication.     

A Motion-Stabilized W-Band Radar for Shipboard Observations of Marine Boundary-Layer Clouds

Cloud radars at X, Ka and W-bands have been used in the past for ocean studies of clouds, but the lack of suitable stabilization has limited their usefulness in obtaining accurate measurements of the velocity structure of cloud particles and the heights of cloud features. A 94 GHz (W-band) radar suitable for use on shipboard studies of clouds has been developed that is small and lightweight and can maintain the radar’s beam pointing in the vertical to reduce the affects of the pitch and roll of the ship. A vertical velocity sensor on the platform allows the effects of the ship’s heave to be removed from the measured cloud particle motions. Results from the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-Rex) field program on the NOAA vessel Ronald H. Brown demonstrate the improvements to the cloud measurements after the ship’s motion effects are removed. The compact design of the radar also makes it suitable for use in aircraft studies. The radar is being repackaged to fit in an aft bay of a NOAA P3 aircraft to observe sea-spray profiles during ocean storms.     

On the number concentration of aerosols in towns

A simple model of the aerosol plume generated by a town is developed under the assumption that the generation rate of particles is proportional to the town population. Thus the volume concentration of aerosols in towns is predicted for various meteorological conditions. The effects of localized sources of aerosols, such as industrial chimney stacks, are considered.     

The annual cycle of peroxides and ozone in marine air at Cape Grim, Tasmania

The concentration of gas-phase peroxides has been measured almost continuously at the Cape Grim baseline station (41° S) over a period of 393 days (7702 h of on-line measurements) between February 1991 and March 1992. In unpolluted marine air a distinct seasonal cycle in concentration was evident, from a monthly mean value of>1.4 ppbv in summer (December) to <0.2 ppbv in winter (July). In the summer months a distinct diurnal cycle in peroxides was also observed in clean marine air, with a daytime build-up in concentration and decay overnight. Both the seasonal and diurnal cycles of peroxides concentration were anticorrelated with ozone concentration, and were largely explicable using a simple photochemical box model of the marine boundary layer in which the central processes were daytime photolytic destruction of ozone, transfer of reactive oxygen into the peroxides under the low-NO_x ambient conditions that favour self-reaction between peroxy radicals, and continuous heterogeneous removal of peroxides at the ocean surface. Additional factors affecting peroxides concentrations at intermediate timescales (days to a week) were a dependence on air mass origin, with air masses arriving at Cape Grim from higher latitudes having lower peroxides concentrations, a dependence on local wind speed, with higher peroxides concentrations at lower wind speeds, and a systematic decrease in peroxides concentration during periods of rainfall. Possible physical mechanisms for these synoptic scale dependencies are discussed.     

Detection of Cyclic Patterns Using Wavelets: An Example Study in the Ormskirk Sandstone, Irish Sea

This study shows how wavelet analysis can be used on well log and drill core data to identify cyclicity in sedimentary sequences. Three possible methods for determining wavelength were investigated: the Morlet wavelet, the Fourier transform, and the semivariogram. When applied to several hypothetical signals similar to those observed in petrophysical measurements in hydrocarbon reservoirs, all three methods could identify the presence of cyclicity. Only the wavelet scalogram, however, gave a clear indication of when the cyclic element was present and where frequency changes occurred in the signal. To illustrate the wavelet analysis, we processed well log and core data from a well in the Ormskirk Sandstone and determined the wavelet coefficients for each zone and the wavelengths of the strongest cyclicities. The cyclicities observed corresponded well with sedimentary features of the formation (e.g., channels and channel sets). Also, ratios of the cyclicity wavelengths corresponded with ratios of the Milankovitch precession, obliquity, and eccentricity periods. This result is in agreement with other investigators, who have proposed that Milankovitch-driven climate changes exercised an important control on Ormskirk Sandstone deposition.     

A system-level modelling perspective of the KwaZulu-Natal Bight ecosystem,eastern South Africa

The KwaZulu-Natal Bight comprises the only sizeable shelf region on the eastern coast of South Africa, and is influenced by both the Agulhas Current on its seaward side and rivers and estuaries on its landward side. Established knowledge of the effect of the Agulhas Current includes the influence on nutrient concentrations in the bight of a semi-permanent upwelling cell at its northern border (St Lucia) and, to a much lesser extent, of a semi-permanent eddy feature at its southern extremity. Current modelling efforts, however, point to a very important role of land-derived nutrients, which supplement the productivity of food webs of the bight. This connectivity of the bight to its adjacent ecosystems has various implications. First, its productivity has traditionally been viewed via phytoplankton growth, whereas ecosystem modelling efforts point to a very high reliance on imported detritus (mainly land-derived) in order to sustain especially the rich benthic food web. The benthos in the bight dominates the food web, and is in marked contrast to the upwelling system of the west coast of South Africa (Atlantic Ocean) where water-column productivity dominates. Second, the importance of the connectivity of the Thukela Bank prawn-trawling ground to estuarine nursery areas, which has been modelled quantitatively, highlights the significance of this particular ecosystem connectivity for fisheries and also for the Thukela Bank food web. Heterogeneity across the bight is apparent for nutrient turnover rates (carbon, nitrogen, phosphorus), CNP content and stoichiometry, whereas nitrogen is a limiting nutrient across the entire bight. The food web near the Thukela River is richer in nutrient content and more active (higher turnover rates) compared to the northern and southern parts of the bight. This environmental heterogeneity was also apparent from the CNP content and stoichiometry of the various species and species groups in the bight. Requirements to take the hydrodynamic, biogeochemical and first ecosystem modelling efforts towards a meaningful predictive capability are discussed. The importance of adopting a system-level view of the bight and its connected systems for realistic exploration of global change scenarios is highlighted.     

Ozone in the Marine Boundary Layer at Cape Grim: Model Simulation

A photochemical box model has been used to simulate the mixing ratio ofozone under conditions reflecting those encountered in the marine boundarylayer at Cape Grim, Tasmania, where a decade-long record of ozone mixingratio is available. The model is based on the proposition that ozone loss byphotolysis, atmospheric reaction with hydroperoxy and hydroxyl radicals, andsurface deposition is balanced by ozone gain via entrainment from the lowerfree troposphere with a small additional source in summer from photolysis ofnitrogen dioxide. This model simulates very well the observed ozone records,reproducing both the small diurnal cycle in ozone mixing ratio observedduring the summer months, and the factor of two seasonal ozone cycle showinga distinct winter maximum and summer minimum. The model result confirms thatunder the low-NO_x conditions of the clean marine boundarylayer net photochemical loss of ozone occurs at all times of year.