John F. Dower

The Role of Microscale Turbulence in the Feeding Ecology of Larval Fish

Publisher Summary The chapter discusses the role of microscale turbulence in the feeding ecology of larval fish. The chapter reviews the development and application of the term “turbulence theory” specifically as it relates to the feeding ecology of larval fish. Turbulent motions are produced when local buoyancy and/or shear forces generate instabilities in local pressure and density fields. The chapter focuses on the interactions between individual larval fish and their prey that restricts the discussion primarily to millimetre-scale turbulent motions. The chapter presents a brief review of the physics of oceanic turbulence, and traces the development of the original Rothschild and Osborn model. The chapter elaborates on more recent attempts to model the effects of microscale turbulence on encounter rates. The chapter also reviews both empirical and field studies designed to test various elements of the turbulence theory. Finally, the chapter concludes by identifying unresolved questions and suggests some specific lines of investigation designed to further enhance the understanding of small-scale physical-biological coupling. The chapter also made reference to the parallel research on the importance of microscale turbulence to the feeding ecology of zooplankton and protozoans.

Observations of Biologically Generated Turbulence in a Coastal Inlet

Measurements in a coastal inlet revealed turbulence that was three to four orders of magnitude larger during the dusk ascent of a dense acoustic-scattering layer of krill than during the day, elevating daily-averaged mixing in the inlet by a factor of 100. Because vertically migrating layers of swimming organisms are found in much of the ocean, biologically generated turbulence may affect (i) the transport of inorganic nutrients to the often nutrient-depleted surface layer from underlying nutrient-rich stratified waters to affect biological productivity and (ii) the exchange of atmospheric gases such as CO2 with the stratified ocean interior, which has no direct communication with the atmosphere.

A strong biological response to oceanic flow past Cobb Seamount

We report results of a CTD and chlorphyll a survey from Cobb Seamount, a shallow seamount in the northeast Pacific. Our results show a several-fold increasein the standing crop of chlorophyll a is centred over the seamount. Current meter and drifter data indicate an anticyclonic deflection of deep currents around consistent with a theoretical stratified Tylor cone. Cobb differs from other seamounts where similar phenomena have been reported (Owens and Hogg; 1980, Deep-Sea Research, 27, 1029–1045; Gouldet al., 1981, Deep-Sea Research, 28, 409–440; Genin and Boehlert, 1985, Journal of Marine Research, 43, 907–924) in that its summit penetrates well into the euphotic zone. A Taylor column existing at such shallow depths could locally enhance primary production, providing a significant source of energy for higher trophic levels on the seamount. Indirect evidence for such a scenario comes from observations of a high biomass benthic community on Cobb Seamount.

“Seamount effects” in the zooplankton community near Cobb Seamount

Abstract Oceanic seamounts often support large nektonic stocks. Since the mid-1950s it has been believed that this high productivity results, in part, from biological response to the physical interaction between oceanic currents and the abrupt topographic profiles represented by most seamounts. The “classic theory” for the production/maintenance of seamount nektonic stocks suggests that (i) the combination of localized upwelling and the trapping/concentrating action of closed anticyclonic vortices (i.e. Taylor cones) enhance local primary production, (ii) thereby promoting local secondary productivity that, (iii) supports local nektonic populations. Here we test one element of this theory: whether proximity to a shallow seamount is associated with changes in zooplankton abundance and species composition. Zooplankton samples were collected near Cobb Seamount, a shallow ( 100 m and is not an effective retention mechanism. Total zooplankton abundance did not vary significantly on- versus off-seamount. However, using a variety of nonparametric multivariate techniques we demonstrate that a “seamount effect” on zooplankton-community composition is detectable up to 30 km from the seamount summit. This effect is superimposed on (and locally much stronger than) the expected slow decline in resemblance as between-sample geographic distance increases. Possible mechanisms by which this effect operates include: differential growth or reproduction, differential mortality and behavioral or migratory effects. The on-off seamount differences are accounted for largely by the increased relative abundances of two fast-growing opportunists, doliolids (Dolioletta sp) and larvaceans (Oikopleura sp.), near Cobb Seamount. Predation pressure from seamount fish and active avoidance of the seamount by zooplankton may also play a role in generating the seamount effect. The absence of an effective trapping mechanism and the fact that total zooplankton abundance does not increase near the seamount lead us to conclude that the bottom-up model of localized energy transfer proposed under the “classic hypothesis” is incorrect for Cobb Seamount: nektonic stocks at Cobb Seamount (and, possibly, other shallow seamounts) are more likely supported by flow-through (i.e. advected) rather than local production.

Reconstruction of environmental histories to investigate patterns of larval radiated shanny ( Ulvaria subbifurcata ) growth and selective survival in a large bay of Newfoundland

We used otolith microstructure analysis to reconstruct the growth histories of larval radiated shanny ( Ulvaria subbifurcata ) collected over a 2-week period in Trinity Bay, Newfoundland. A dynamic 3-dimensional, eddy-resolving circulation model of the region provided larval drift patterns, which were combined with measurements of temperature and zooplankton abundance to assess the environmental history of the larvae. The abundance of juvenile and adult capelin ( Mallotus villosus ), the dominant planktivorous fish in this area, was monitored using five hydroacoustic surveys. The goal was to determine whether environmental histories are helpful in explaining spatial and temporal differences in larval shanny growth, measured as cumulative distribution functions (CDF) of growth rates. We found evidence for a selective loss of slower growing individuals and recognized considerable spatial differences in the CDF of larval growth rates. Consistent patterns in capelin abundance suggested that faster growing survivors, sampled at the end of the 2-week period, developed in areas of low predator densities. A dome-shaped relationship between temperature and larval growth was observed, explaining a significant but small amount of the overall variability (14%). Effects of experienced prey concentrations on larval growth rates could not be demonstrated.

The Protozooplankton-Ichthyoplankton Trophic Link: An Overlooked Aspect of Aquatic Food Webs

ABSTRACT. Since the introduction of the microbial loop concept, awareness of the role played by protozooplankton in marine food webs has grown. By consuming bacteria, and then being consumed by metazooplankton, protozoa form a trophic link that channels dissolved organic material into the “classic” marine food chain. Beyond enhancing energy transfer to higher trophic levels, protozoa play a key role in improving the food quality of metazooplankton. Here, we consider a third role played by protozoa, but one that has received comparatively little attention: that as prey items for ichthyoplankton. For >100 years it has been known that fish larvae consume protozoa. Despite this, fisheries scientists and biological oceanographers still largely ignore protozoa when assessing the foodweb dynamics that regulate the growth and survival of larval fish. We review evidence supporting the importance of the protozooplankton–ichthyoplankton link, including examples from the amateur aquarium trade, the commercial aquaculture industry, and contemporary studies of larval fish. We then consider why this potentially important link continues to receive very little attention. We conclude by offering suggestions for quantifying the importance of the protozooplankton–ichthyoplankton trophic link, using both existing methods and new technologies.

On Turbulence Production by Swimming Marine Organisms in the Open Ocean and Coastal Waters

Abstract Microstructure and acoustic profile time series were collected near Ocean Station P in the eastern subarctic North Pacific and in Saanich Inlet at the south end of Vancouver Island, British Columbia, Canada, to examine production of turbulent dissipation by swimming marine organisms. At Ocean Station P, although a number of zooplankton species are large enough to generate turbulence with Reynolds numbers Re > 1000, biomass densities are typically less than 103 individuals per cubic meter (<0.01% by volume), and turbulent kinetic energy dissipation rates e were better correlated with 16-m vertical shear than acoustic backscatter layers. In Saanich Inlet, where krill densities are up to 104 individuals per cubic meter (0.1% by volume), no dramatic elevation of dissipation rates e was associated with dusk and dawn vertical migrations of the acoustic backscatter layer. Dissipation rates are a factor of 2 higher [〈e〉 = 1.4 × 10−8 W kg−1, corresponding to buoyancy Re = 〈e〉/(νN 2) ∼ 140] in acoustic bac...

INVASION DYNAMICS OF THE VARNISH CLAM (NUTTALLIA OBSCURATA): A MATRIX DEMOGRAPHIC MODELING APPROACH

Marine invaders have become a significant threat to native biodiversity and ecosystem function. In this study, the invasion of the varnish clam (Nuttallia obscurata) in British Columbia, Canada, is investigated using a matrix modeling approach to identify the life history characteristics most crucial for population growth and to investigate population differences. Mark-recapture analyses and field collections from 2003 to 2004 were used to determine individual growth, survival rates, and fecundity for two sites. A multi-state matrix model was used to determine population growth rates and to conduct sensitivity and elasticity analyses. A life table response experiment was also used to determine what life history stage contributed most to observed differences in population growth rates. Population survey data were used in conjunction with the matrix model to determine plausible recruitment levels and to investigate recruitment scenarios. Both populations are currently declining but are likely sustainable because of the pulsed nature of large recruitment events. Survival of larger clams (>40 mm) is the most important for population growth based on elasticity and sensitivity analyses. Adult survival also had the largest influence on observed differences between site-specific population growth rates. The two populations studied differed in recruitment dynamics; one experiencing annual recruitment with higher post-settlement mortality and the other, episodic recruitment and lower post-settlement mortality. The most influential factor for the successful invasion of the varnish clam appears to be survival of the larger size classes. Therefore, any process that decreases adult survival (e.g., predation, commercial harvest) will have the greatest impact on population growth.