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In situ measurements of acoustic target strengths of gas-bearing siphonophores.-ICES Journal of Marine Science, 58: 740-749 Acoustic target strengths of free-swimming siphonophores were measured in situ at 24 and 120 kHz from a remotely operated vehicle equipped with both acoustic transducers and a video camera. The transducers and camera were co-registered by aiming both instruments at the same volume of water and time-stamping the recorded data. The video system allowed us to search for and identify siphonophores, and verified whether individual animals were centered in, or near, the axis of the acoustic beams. A towed, down-looking acoustic survey system (operating at 120 kHz) measured the target and volume scattering strengths of scattering layers, presumed to be dominated by siphonophores. Spatial density of the sound scatterers was estimated from survey data. Our results confirm that free-swimming physonect siphonophores have relatively high acoustical target strengths caused by a gas inclusion in the pneumatophore of each animal. A relatively small number of these animals can dominate the backscattering detected by acoustic surveys even though other taxa may dominate the plankton on a numerical or biomass basis. Siphonophore colonies are fragile and cannot be reliably censused with nets. Our estimates of siphonophore target strengths can improve the ability to use acoustics to quantitatively census siphonophores and other taxa possessing comparably-sized gas inclusions.

The northern Gulf of Mexico (NGOM) is one of the well-studied areas of global ocean, yet direct estimates of upper ocean particulate organic carbon (POC) fluxes from this region are limited. The present work reports vertical fluxes of POC from the oligotrophic region of NGOM utilizing short-lived radionuclide pairs 234 Th/ 238 U and 210 Po/ 210 Pb. In spite of the difference in time scale both 210 Po and 234 Th based estimates are in reasonable agreement with sinking POC fluxes, caught in sediment traps. POC flux estimates ranged between 22-41 mg C m-2 day-1 at 150 m and 6-40 mg C m-2 day-1 at 250 m. The average export efficiency at base of euphotic zone (E z) was found to be 0.07±0.03 while the export ratio (T 100) at 100m below euphotic zone was found to be 0.66±0.18 indicating that most of the attenuation of NPP in this region is set in the surface layer (low E z ratio) and there is relatively little flux attenuation in the subsurface (high T 100) which is typical for oligotrophic settings. Satellite based export efficiencies predicted by the Laws and Dunne models are on average found to be two times higher than the in situ observations while estimates from the Henson model were found to be similar or lower. This is consistent with the observation on a global scale where we find export estimates from in situ data to be consistently lower than those predicted by the Laws export model for the temperature range of 20-25 ºC. The discrepancy between modelled estimates and in situ measurements of POC fluxes highlights the fact that global empirical models of satellite based POC fluxes that only consider temperature are overly simple and may need further refinement for ocean biome specific scaling to accurately predict export fluxes.

Fishery management production models tend to stress only the elements directly linked to fish (i.e. fish, fish food, and fish predators). Large coastal jellyfish are major consumers of plankton in heavily fished ecosystems; yet, they are frequently not included as model components. We explore the relationship between gulf menhaden (Brevoortia patronus) and the large scyphozoan jellyfish (Aurelia spp. and Chrysaora sp.), and provide an examination of trophic energy transfer pathways to higher trophic levels in the northern Gulf of Mexico. A trophic network model developed within the ECOPATH framework was transformed to an end-to-end model to map foodweb energy flows. Relative changes in functional group productivity to varying gulf menhaden consumption rates, jellyfish consumption rates, and forage fish (i.e. gulf menhaden, anchovies, and herrings) harvest rates were evaluated within a suite of static, alternative energy-demand scenarios using ECOTRAN techniques. Scenario analyses revealed forage fish harvest enhanced jellyfish productivity, which, in turn, depressed menhaden productivity. Modelled increases in forage fish harvest caused pronounced changes in ecosystem structure, affecting jellyfish, marine birds, piscivorous fish, and apex predators. Menhaden were found to be a more efficient and important energy transfer pathway to higher trophic levels compared with jellyfish. A simulated increase in jellyfish abundance caused the relative production of all model groups to decline. These outcomes suggest that jellyfish blooms and forage fish harvest have demonstrable effects on the structure of the northern Gulf of Mexico ecosystem.

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