Influence of buoyancy and vertical distribution of sardine Sardinops sagax eggs and larvae on their transport in the northern Benguela ecosystem

In recent years, sardine Sardinops sagax spawning has been recorded inshore off central Namibia. Field observations on eggs and laboratory measurements show that spawning, demonstrated by the distribution of newly spawned eggs, takes place just below the upper mixed layer. The high positive buoyancy of the eggs causes them to ascend rapidly to the surface layer, where they are moved offshore by upwelling-induced offshore transport. However, increased wind-induced mixing also influences the vertical distribution of eggs, causing them to be partly mixed down below the layer moving offshore and into the layer moving inshore. This mechanism acts to retard the transport and offshore loss of eggs from the spawning areas. The vertical distribution of sardine larvae, with highest concentrations deeper than 20 m, indicates active movement out of the layer moving offshore, and this tendency seems to be more pronounced for older larvae. Hence, vertical migration of larvae is an additional factor mitigating their loss from nearshore. Taken together, these features seem to minimize the offshore loss of offspring, particularly in periods of low stock biomass when spawning close to the shore seems to be common.Keywords: buoyancy, northern Benguela, sardine, vertical distributionAfrican Journal of Marine Science 2001, 23: 85–9

Interaction between Cape hake spawning and the circulation in the northern Benguela upwelling ecosystem

Cape hake in Namibian waters are demersal and mesopelagic spawners, spawning peaking offshore between 100 and 400 m deep, depending on local environmental conditions. The cross-shelf circulation, low-oxygen layers and mesoscale gyres are three important environmental factors influencing hake spawning behaviour and subsequent transport of the spawning products. Normally, hake spawn offshore near the bottom at depths of 150–400 m. However, during one cruise, spawning was concentrated below several subsurface mesoscale gyres, resulting in reduced dispersion of the eggs and larvae. When the low-oxygen layer above the bottom is pronounced, hake spawning has been observed close to the top of the layer at oxygen concentrations as low as 0.2–0.3 ml l-1. The relatively small size of the eggs and their high specific gravity make them ascend quite slowly from the spawning depths, 10–40 m per day. Consequently, hake eggs spawned deeper than 200 m hatch before they reach the upper mixed layer. The newly hatched larvae are relatively undeveloped, without functional eyes or mouth, and display little swimming activity during their first hours, but laboratory observations have revealed subsequent periods of downward swimming activity. Based on current field observations, on buoyancy measurements of eggs and larvae and on observed larval behaviour, it is concluded that hake eggs and larvae are transported onshore by features of the upwelling subsurface circulation that compensate for offshore movement of surface water. This may be the basic mechanism concentrating early juvenile hake nearshore. Spawning activity near the low-oxygen layer might be a behavioural adaptation to minimize egg predation, because few other species are expected to survive such low concentrations of oxygen.Keywords: buoyancy, eggs, larvae, recruitment processes, retention, vertical distributionAfrican Journal of Marine Science 2001, 23: 317–33

Use of voltammetric solid-state (micro)electrodes for studying biogeochemical processes: Laboratory measurements to real time measurements with an in situ electrochemical analyzer (ISEA)

Solid-state voltammetric (micro)electrodes have been used in a variety of environments to study biogeochemical processes. Here we show the wealth of information that has been obtained in the study of sediments, microbial mats, cultures and the water column including hydrothermal vents. Voltammetric analyzers have been developed to function with operator guidance and in unattended mode for temporal studies with an in situ electrochemical analyzer (ISEA). The electrodes can detect the presence (or absence) of a host of redox species and trace metals simultaneously. The multi-species capacity of the voltammetric electrode can be used to examine complex heterogeneous environments such as the root zone of salt marsh sediments. The data obtained with these systems clearly show that O2 and Mn2+ profiles in marine sedimentary porewaters and in microbial biofilms on metal surfaces rarely overlap indicating that O2 is not a direct oxidant for Mn2+. This lack of overlap was suggested originally by Joris Gieskes\u27 group. In waters emanating from hydrothermal vents, Fe2+, H2S and soluble molecular FeS clusters (FeSaq) are detected indicating that the reactants for the pyrite formation reaction are H2S and soluble molecular FeS clusters. Using the ISEA with electrodes at fixed positions, data collected continuously over three days near a Riftia pachyptila tubeworm field generally show that O2 and H2S anti-correlate and that H2S and temperature generally correlate. Unlike sedimentary environments, the data clearly show that Riftia live in areas where both O2 and H2S co-exist so that its endosymbiont bacteria can perform chemosynthesis. However, physical mixing of diffuse flow vent waters with oceanic bottom waters above or to the side of the tubeworm field can dampen these correlations or even reverse them. Voltammetry is a powerful technique because it provides chemical speciation data (e.g.; oxidation state and different elemental compounds/ions) as well as quantitative data. Because (micro)organisms occupy environmental niches due to the system\u27s chemistry, it is necessary to know chemical speciation. Voltammetric methods allow us to study how chemistry drives biology and how biology can affect chemistry for its own benefit

A Tribute to William S. Geimer

A Tribute to William S. Geimer Meredith Susan Palmer* * Associate Dean for Student Affairs and Admissions, Washington and Lee University School of Law, J.D., 1985, Washington and Lee University. Truly, I say to you, as you did it to one of the least of these my brethren, you did it to me.1 Bill Geimer is not a man of material things. But somehow, in the material things that he chose to have around him - the mementos, the odd souvenirs, the offerings from students, clients, friends - I find the story of the man, the teacher, and the lawyer Bill Geimer is to me. In my mind's eye, I see Bill's office here at Washington and Lee as I first saw it, as a hesitant first-year law student approaching the sanctum sanctorum of a faculty member. There is a desktop name plate, regular Army issue, decorated with grenades or something equally militaristic and intimidating. But next to that is a picture of Dr. Martin Luther King, with his stirring words evoking hope for justice and freedom on earth. And next to that, a membership certificate for the Lawyer's Alliance for Nuclear Arms Control, and a drawing of Quijote, lance raised to a windmill. How could this be? While reconciling the seemingly irreconcilable is the everyday task of the law student, no trick of logic could make these disparate pieces fit; one simply had to get to know the man. That one could do so as a law student is itself part of what defines Bill Geimer. The student who came to know Bill heard a story of a kid from the rural south, off to a small state university to play basketball but who, like countless kids before him, spent a bit more time playing than studying. Come Graduation Day, with no game plan, he joined Uncle Sam's family. A grounding in tanks, guns, and tactics preceded duty with the military police, but something else happened during those years. Assigned to assist with an investigation, he finds an aptitude for the law, an appreciation of justice, and a burning sense of the unfairness of justice denied. The young man who returned from service to go on to a distinguished career as a law student at the University of North Carolina was a man with a game plan, and a mission. It was no surprise that Bill's mission as a lawyer was not in the office towers of Atlanta or the corporate boardrooms of Wall Street, but among the plain folks of Fayetteville, North Carolina, and later in the community of migrant farm labor working and living hard under the hot Carolina sun. I see another memento, this time in Bill's home. It's an odd piece of, well, let's call it folk art. Crudely carved, frankly ugly. Taste is a delicate thing, so one observes diplomatically that it is an "interesting" object. There's a story, of course, a story of a client for whom Bill had worked, putting together the paperwork to get a fledgling arts-and-crafts shop off the ground. This client, like many Bill served, claimed not to have the cash to pay. But, the client said, he could give Bill this valuable piece of work. Barter being a fundamentally more honest exchange and thus appealing to Bill, the transaction was concluded. Not until Bill brought his sculpture home did he discover the price tag stuck to the underside, for something substantially less than the agreed-upon fee. He kept the awkward thing, price tag intact, I think as a reminder of his own fallibility, of the ingenuity of the ordinary man, and of the need to forget neither. Another memento, in some ways the most important to this former student, is a large, faded, well-handled poster on the professor's office door. A young man in Carolina Blue goes up for a shot, stretching for the basket, with seconds left on the clock. Anyone who knows Bill Geimer knows that he is devoted to Carolina basketball. In fact, Bill may have spent so much time watching the Tar Heels play that he began to confuse teaching with coaching. As a former student of Bill's, I'm not sure that was a bad thing.

Recruitment Variability in North Atlantic Cod and Match-Mismatch Dynamics

Background Fisheries exploitation, habitat destruction, and climate are important drivers of variability in recruitment success. Understanding variability in recruitment can reveal mechanisms behind widespread decline in the abundance of key species in marine and terrestrial ecosystems. For fish populations, the match-mismatch theory hypothesizes that successful recruitment is a function of the timing and duration of larval fish abundance and prey availability. However, the underlying mechanisms of match-mismatch dynamics and the factors driving spatial differences between high and low recruitment remain poorly understood. Methodology/Principal Findings We used empirical observations of larval fish abundance, a mechanistic individual-based model, and a reanalysis of ocean temperature data from 1960 to 2002 to estimate the survival of larval cod (Gadus morhua). From the model, we quantified how survival rates changed during the warmest and coldest years at four important cod spawning sites in the North Atlantic. The modeled difference in survival probability was not large for any given month between cold or warm years. However, the cumulative effect of higher growth rates and survival through the entire spawning season in warm years was substantial with 308%, 385%, 154%, and 175% increases in survival for Georges Bank, Iceland, North Sea, and Lofoten cod stocks, respectively. We also found that the importance of match-mismatch dynamics generally increased with latitude. Conclusions/Significance Our analyses indicate that a key factor for enhancing survival is the duration of the overlap between larval and prey abundance and not the actual timing of the peak abundance. During warm years, the duration of the overlap between larval fish and their prey is prolonged due to an early onset of the spring bloom. This prolonged season enhances cumulative growth and survival, leading to a greater number of large individuals with enhanced potential for survival to recruitment