Effects of variable winds on biological productivity on continental shelves in coastal upwelling systems

The production and distribution of biological material in wind-driven coastal upwelling systems are of global importance, yet they remain poorly understood. Production is frequently presumed to be proportional to upwelling rate, yet high winds can lead to advective losses from continental shelves, where many species at higher trophic levels reside. An idealized mixed-layer conveyor (MLC) model of biological production from constant upwelling winds demonstrated previously that the amount of new production available to shelf species increased with upwelling at low winds, but declined at high winds [Botsford, L.W., Lawrence, C.A., Dever, E.P., Hastings, A., Largier, J., 2003. Wind strength and biological productivity in upwelling systems: an idealized study. Fisheries Oceanography 12, 245–259]. Here we analyze the response of this model to time-varying winds for parameter values and observed winds from the Wind Events and Shelf Transport (WEST) study region. We compare this response to the conventional view that the results of upwelling are proportional to upwelled volume. Most new production per volume upwelled available to shelf species occurs following rapid increases in shelf transit time due to decreases in wind (i.e. relaxations). However, on synoptic, event time-scales shelf production is positively correlated with upwelling rate. This is primarily due to the effect of synchronous periods of low values in these time series, paradoxically due to wind relaxations. On inter-annual time-scales, computing model production from wind forcing from 20 previous years shows that these synchronous periods of low values have little effect on correlations between upwelling and production. Comparison of model production from 20 years of wind data over a range of shelf widths shows that upwelling rate will predict biological production well only in locations where cross-shelf transit times are greater than the time required for phytoplankton or zooplankton production. For stronger mean winds (narrower shelves), annual production falls below the peak of constant wind prediction [Botsford et al., 2003. Wind strength and biological productivity in upwelling systems: an idealized study. Fisheries Oceanography 12, 245–259], then as winds increase further (shelves become narrower) production does not decline as steeply as the constant wind prediction.     

A 15-month study of zooplankton ingestion by farmed mussels (Mytilus edulis) in Bantry Bay, Southwest Ireland

There is a growing body of evidence to suggest that bivalve molluscs routinely ingest zooplankton. To elucidate further these observations, a 15-month study of zooplankton ingestion by farmed mussels was conducted using mussel long-lines in Bantry Bay, Ireland. Stomach content analysis of the mussels showed that there was evidence of zooplankton ingestion throughout the sampling period, but that highest mean numbers of zooplankters were ingested by mussels in the spring and summer months. Various zooplankton species were present in mussel stomachs. Harpacticoid copepods were found more often in stomach contents than calanoid copepods, probably due to their proximity to the bivalves' inhalent siphons. Barnacle cyprids featured in large numbers in stomach contents, but only for a period of 3 months which broadly corresponded with their pelagic phase. Sizes of ingested zooplankton ranged from 126 μm to 6 mm, but more of the smaller zooplankters (e.g. crustacean nauplii) were ingested. When lengths of ingested copepods were compared with those found in plankton net samples, it was found that the net-sampled copepods were significantly larger than those found in mussel stomachs, suggesting that mussels select for smaller categories within the zooplankton available to them. Soft bodied zooplankton was rarely found in mussel stomachs but their absence may be due to rapid digestion or they may have been destroyed in the preservation process. Ingestion of zooplankton by bivalves is discussed in the context of the impacts mussel farms have on resident zooplankton populations.     

A note on ocean surface drift with application to surface velocities measured with HF Radar

The ocean drift current consists of a (local) pure drift current generated by the interaction of wind and waves at the sea surface, to which the surface geostrophic current is added vectorially. We present (a) a similarity solution for the wave boundary layer (which has been validated through the prediction of the 10-m drag law), from which the component of pure drift current along the direction of the wind (and hence the speed factor) can be evaluated from the 10-m wind speed and the peak wave period, and (b) a similarity solution for the Ekman layers of the two fluids, which shows that under steady-state neutral conditions the pure drift current lies along the direction of the geostrophic wind, and has a magnitude 0.034 that of the geostrophic wind speed. The co-existence of these two similarity solutions indicates that the frictional properties of the coupled air-sea system are easily evaluated functions of the 10-m wind speed and the peak wave period, and also leads to a simple expression for the angle of deflection of the pure drift current to the 10 m wind. The analysis provides a dynamical model for global ocean drift on monthly and annual time scales for which the steady-state neutral model is a good approximation. In particular, the theoretical results appear to be able to successfully predict the mean surface drift measured by HF Radar, which at present is the best technique for studying the near surface velocity profile.     

Appraisal of storm beach buffer width for cyclonic waves

The loss of beach sand from berm and dune due to high waves and surge is a universal phenomenon associated with sporadic storm activities. To protect the development in a coastal hazard zone, hard structures or coastal setback have been established in many countries around the world. In this paper, the requirement of a storm beach buffer, being a lesser extent landward comparing with the coastal setback to ensure the safety of infrastructures, is numerically assessed using the SBEACH model for three categories of wave conditions in terms of storm return period, median sand grain size, berm width, and design water level. Two of the key outputs from the numerical calculations, berm retreat and bar formation offshore, are then analysed, as well as beach profile change. After having performed a series of numerical studies on selected large wave tank (LWT) test results with monochromatic waves using SBEACH, we may conclude that: (1) Berm erosion increases and submerged bar develops further offshore as the storm return period increases for beach with a specific sand grain size, or as the sand grain reduces on a beach under the action of identical wave condition; (2) Higher storm waves yield a large bar to form quicker and subsequently cause wave breaking on the bar crest, which can reduce the wave energy and limit the extent of the eroding berm; (3) A larger buffer width is required for a beach comprising small sand grain, in order to effectively absorb storm wave energy; and (4) Empirical relationships can be tentatively proposed to estimate the storm beach buffer width, from the input of wave conditions and sediment grain size. These results would benefit a beach nourishment project for shore protection or design of a recreational beach.     

Composition and Useful Properties of Tailings of Marine Manganese Nodules and Crusts

ABSTRACT

Marine manganese nodules and crusts, when processed, yield tailings which may be utilized for environmental and economic benefit. The key to the reasonable and effective utilization of these tailings lies in making a systematic appraisal of their composition and properties. This article gives an introduction to the investigation of manganese tailings properties. The tailings have a high iron and/or manganese content, high surface area, high porosity, and fine grain size. Some tailings have a high rare earth element content which is valuable. They may also have high SO₃, arsenic, and uranium contents which are harmful. Depending on the process used to produce the tailings, there will likely be some differences in chemical composition, mineral assemblages, surface area and adsorption capability, pore diameter and volume, density and pH. In assigning potentially beneficial applications for these tailings, these differences should be taken into account to optimize utilization.     

Relationship between turbidity and fish diets in Lakes Waahi and Whangape,New Zealand

Abstract

Stomach contents of Gobiomorphus cotidianus,Retropinna retropinna, Gambusia affinis, and Anguilla australis were compared between two shallow lakes in the lower Waikato River basin, to examine the relationship between turbidity and diet. Lake Waahi and the south arm of Lake Whangape had been turbid (20–40 g suspended solids (SS) m^?3) and devoid of submerged macrophytes since the late 1970s and early 1980s, respectively. The main basin of Lake Whangape had been generally clearer (5 g SS m^?3) with dense beds of submerged macrophytes, but at the time of sampling (1987) water clarity had deteriorated (> c. 10 g SS m³) and submerged macrophytes had declined. The mysid Tenagomysis chiltoni was an important prey for all species of fish from turbid water bodies but was less important in stomachs of fish in the main basin of Lake Whangape. Apparently, mysids were not an important prey in Lake Waahi before it became turbid. Chironomid larvae and pupae dominated the diets of small fish in the main basin of Lake Whangape. Fish and mysids were the most important prey of shortfinned eels in both lakes, with mysids most important in Lake Waahi. High mysid densities in the turbid water bodies provide an alternative food resource apparently compensating for those lost by fish when water clarity declined and submerged macrophytes collapsed.     

Land‐use impacts and water quality targets in the intensive dairying catchment of the Toenepi Stream,New Zealand

Water quality monitoring in Toenepi Stream, New Zealand, started in 1995 in a study of dairy farming influences on lowland stream quality and has continued since then with brief interruptions. Surveys have provided information about changes in farm and soil management practices as they relate to environmental sustainability. Although average water quality in Toenepi Stream has changed little during 1995–2004, there have been some notable improvements. Water clarity measured by black disc has improved from 0.6m to 1.5m, and median ammonia‐N and nitrate‐N concentrations have declined by 70% and 57%, respectively. The frequency and magnitude of extreme concentrations have declined—most notably for nitrogen (N) forms, which also had decreased mean values. Specific yields for suspended solids (SS) and phosphorus (P) forms in 2002–04 were 47–67% of 1995–97 values, mainly because of lower water yields. Reduced specific yields for N forms in 2002–04 (34–37% of 1995–97 yields) were also attributable to lower mean concentrations in stream water. Faecal bacteria concentrations have not abated and are on average 2–3 times recommended guideline values for contact recreation. Fewer dairy farms and an increased proportion irrigating dairyshed effluent to land, rather than discharging it to the stream via two‐pond systems, were likely causes of improvement in water quality. Water quality targets were developed for Toenepi Stream to achieve contact recreation criteria for the Piako River (downstream) and for intrinsic habitat values for Toenepi Stream. A range of mitigation measures has been formulated to meet these targets, but substantial uptake of sustainable farming practices is needed to improve water quality in Toenepi Stream.     

Sedimentary processes and trace metal contaminants in the buccaneer oil/gas field,northwestern gulf of Mexico

Sediments are the ultimate sink for contaminants in the marine environment, and physical processes of sedimentation influence the distribution and accumulation of these contaminants. Evaluation of contaminant levels in sediments is one approach to assessing environmental impact; data interpretation depends on consideration of sediment texture and mineralogy, however, which profoundly influence chemical composition. In this study, comparison of potentially contaminated sediments from the production field with control populations was done only within the context of similar (as to texture and organic carbon and carbonate content) sample groups as determined by cluster analysis. Ba, Cd, and Sr are identified as contaminants. Supported by the identification of a well-crystallized expandable clay—possibly bentonite—drilling fluids are a potential source of Ba. Ba and Sr may be unnaturally high because of their abundance in discharged produced formation waters, but may also be naturally controlled by the unique faunal assemblage associated with the structures. Cd is probably derived from corrosion of the structures and assorted debris on the seafloor. In general, contamination is limited to an area within 100 m of the platforms. Furthermore, substantial erosion around platforms has probably effectively removed and dispersed the bulk of the contaminants introduced into the marine environment by the offshore exploration/production operations.     

Assessing the Measurement Consistency Between the Jason-2/AMR and SARAL/AltiKa/DFMR Microwave Radiometers Using Simultaneous Nadir Observations

SARAL/AltiKa has a Dual Frequency Microwave Radiometer (DFMR), and Jason-2 has an Advanced Microwave Radiometer (AMR). Both microwave radiometer sensors include a 23.8 GHz primary water sensing channel. The measurement consistencies between DFMR and AMR are important for establishing a consistent altimetry data set between SARAL/AltiKa and Jason-2 in order to accurately assess sea level rise in a long-term time series. This study investigates the measurement consistency in the 23.8 GHz channel between DFMR and AMR at the Simultaneous Nadir Overpasses (SNO's) between the two satellites and also at coldest ocean brightness temperature locations. Preliminary results show that while both instruments show no significant trends over the one year since the launch of SARAL, a consistent relative bias of 2.88 K (DFMR higher than AMR) with a standard deviation of 0.98 K is observed. The relative bias at the lowest brightness temperature from the SNO method (-3.82 K) is consistent with that calculated from coldest ocean method (-3.74 K). The relative bias exhibits strong latitude (and scene temperature) dependency, changing from -3.82 K at high latitudes to -0.92 K near the equator. There also exists an asymmetry between the northern and southern hemisphere. The relative bias increases toward the lower end of brightness temperature.     

Reconstructing historic Glacial Lake Outburst Floods through numerical modelling and geomorphological assessment: Extreme events in the Himalaya

Recession of high‐mountain glaciers in response to climatic change frequently results in the development of moraine‐dammed glacial lakes. Moraine dam failure is often accompanied by the release of large volumes of water and sediment, termed a Glacial Lake Outburst Flood (GLOF). Chukhung Glacier is a small (~3 km²) receding valley glacier in Mt. Everest (Sagarmatha) National Park, Nepal. Unlike many Himalayan glaciers, which possess a thick mantle of supraglacial debris, its surface is relatively clean. The glacier terminus has receded 1.3 km from its maximum Holocene position, and in doing so provided the space for an ice‐contact moraine‐dammed lake to develop. The lake had a maximum volume of 5.5 × 10⁵ m³ and drained as a result of breaching of the terminal moraine. An estimated 1.3 × 10⁵ m³ of material was removed from the terminal moraine during breach development. Numerical dam‐breach modelling, implemented within a Generalised Likelihood Uncertainty Estimation (GLUE) framework, was used to investigate a range of moraine‐dam failure scenarios. Reconstructed outflow peak discharges, including failure via overtopping and piping mechanisms, are in the range 146–2200 m³ s^‐1. Results from two‐dimensional hydrodynamic GLOF modelling indicate that maximum local flow depths may have exceeded 9 m, with maximum flow velocities exceeding 20 m s^‐1 within 700 m of the breach. The floodwaters mobilised a significant amount of material, sourced mostly from the expanding breach, forming a 300 m long and 100 m wide debris fan originating at the breach exit. moraine‐dam. These results also suggest that inundation of the entire floodplain may have been achieved within ten minutes of initial breach development, suggesting that debris fan development was rapid. We discuss the key glaciological and geomorphological factors that have determined the evolution of a hazardous moraine‐dammed lake complex and the subsequent generation of a GLOF and its geomorphological impact. © 2014 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.