WROBLEWSKI ET AL.: SURVIVAL OF NORTHERN ANCHOVY LARVAE 



of phytoplankton in the water column, so that 

 the simulated profile of phytoplankton (Fig. la) 

 more closely resembles the observations off 

 Southern California reported in Cullen et al. 



(1983) and Mulhn et al. (1985). 



The modeUng studies by Wroblewski (1984) 

 and Wroblewski and Richman (1987) examined 

 how the concentration of prey responds to per- 

 turbations in the physical oceanographic envi- 

 ronment caused by wind forcing. Wroblewski 



(1984) used scale analysis to deduce the thick- 

 ness of the layer of prey which could be main- 

 tained during wind-induced turbulent mixing. 

 He found that the effectiveness of turbulence in 

 dispersing food for northern anchovy larvae is 

 lowered by any ability of the prey to aggregate 

 into patches. The conclusion was that first-feed- 

 ing larvae could find sufficient concentrations of 

 motile G. splendens in the pycnocline during 

 moderate wind conditions. 



Wroblewski and Richman (1987) coupled the 

 plankton equations to a simplified model of 

 mixed layer dynamics (Niiler 1975) to calculate 

 wind-driven deepening of the mixed layer and 



( |j.g-atom N / '' ) 

 .0123456 



( ng-atom N / "'' ) 

 12 3 4 5 6 



(b) j 



day = 1, MLD = 27m: 



12 3 4 5 6 



(c) 

 80 r day = 2, MLD = 20m 



Figure 1. — Temporal evolution of the one-dimensional (ver- 

 tical) plankton model in response to a single wind-mixing 

 event. Initial conditions are the steady state profiles of 

 phytoplankton (P), zooplankton (Z), and nitrate (N) shown in 

 panel (a). The wind speed during the event is 16 m s"' 

 with 24 h duration. MLD refers to the mixed layer depth. 



the turbulent diffusivity within the mixed layer 

 during and after a vdnd event. Wroblewski and 

 Richman found that wind events are always det- 

 rimental to larval northern anchovy, because 

 wind mixing dissipates vertical structure in prey 

 concentration as Lasker (1975) proposed. How- 

 ever, they discovered that interacting biological 

 and physical processes determine the time inter- 

 val before high concentrations of prey are re- 

 established, i.e., the starvation period endured 

 by the anchovy larvae. Reproduction by prey 

 and their aggi'egation by swimming govern the 

 rate of reestablishment of vertical structure in 

 prey distributions, once wind conditions allow 

 turbulence in the upper water column to dis- 

 sipate. They noted as significant that first-feed- 

 ing larval anchovy forage directly on G. splen- 

 dens and microzooplankton which have the 

 reproductive capacity and migration abiUty to 

 reestabhsh high concentrations shortly after a 

 storm. 



Wroblewski and Richman (1987) were able to 

 quantify the influence of vdnd event magnitude 

 and duration on larval northern anchovy sur- 

 vival. However, as Niiler's (1975) mixed layer 

 model does not permit restratification of the up- 

 per water column by solar heating between wind 

 events, more complex physics was required to 

 explore the influence of interstorm duration on 

 larval anchovy survival. 



Mixed Layer Dynamics 



Here we use the mixed layer dynamics of Mel- 

 lor and Yamada (1974; 1982) which predict both 

 wind-driven deepening of the mixed layer and 

 shallowing of the mixed layer by solar heating. 

 Heat is absorbed at the sea surface and short- 

 wave radiation penetrates the surface, attenuat- 

 ing exponentially in the manner formulated by 

 Simpson and Dickey (1981). 



Klein and Coste (1984) used the turbulence 

 closure scheme of Mellor and Yamada (1974) to 

 study the influence of wind forcing on nutrient 

 transport into the mixed layer, but treated ni- 

 trate as a conservative quantity. Chen et al. 

 (1988) used Mellor and Yamada (1974) dynamics 

 vdth both wind and tidal forcing to simulate ver- 

 tical nutrient mixing in Long Island Sound, but 

 also considered biological consumption and pro- 

 duction of nitrate. We refer the reader to Klein 

 and Coste (1984) and Chen et al. (1988) for de- 

 tails on the implementation of Mellor and 

 Yamada's (1974) level 2.5 dynamics in this type 

 of physical-biological modeUng study. 



389 



