Journal of Plankton Research Volume 2 Number 1 1980 A simulation of the distribution of Acartia clausi during Oregon Upwelling, August 1973 J.S.Wroblewski Department of Oceanography, Dalhousie University, Halifax, Nova Scotia B3H4J1, Canada Downloaded from https://academic.oup.com/plankt/article-abstract/2/1/43/1463810 by Old Dominion University user on 08 July 2019 (Received August 1979; revised November 1979; accepted December 1979) Abstract. The distribution of the estuarine copepod Acartia clausi in coastal waters off Oregon during an upwelling period in August 1973 is simulated. A time dependent, two dimensional (x, z, t) model relates maximum offshore extent of the copepod's four life stages (egg, nauplius, copepodite, and adult) to in- tensity of the wind stress driving the upwelling circulation, stage development time, and mortality. Realistic solutions are obtained by using actual intermittent wind forcing recorded by an anemometer at Newport. Offshore transport is overestimated when the circulation model is driven by theoretical con- tinuous winds, suggesting zooplankton may be washed out of coastal upwelling zones (e.g. off Northwest Africa) which undergo periods of prolonged upwelling. With an accurate model of offshore transport and stage development time, the mismatch between predicted and observed distributions may be used to estimate field mortality of the various stages. Introduction Zooplankton standing stock was once believed to be greater in the slope region off Oregon than over the continental shelf during the summer upwelling season. Peterson (1972) surveyed the oceanic, slope and shelf regions but did not sample the nearshore zone when making this conclusion. In a detailed study of the Oregon up- welling zone, Peterson and Miller (1975) found high concentrations of copepods in the upper 20 m of the water column within 15 km of the coast during the 1969-71 upwelling seasons, as did Myers (1975) in August, 1973. Wroblewski (1977) simulated the abundant zooplankton seaward of the upwelling front but failed to account for the high inshore copepod populations. This research explores the question posed recently by Peterson, et al. (1979): how do high concentrations of zooplankton arise nearshore, and how are they main- tained in the face of offshore transport during coastal upwelling? This paper examines the dynamics of the estuarine species Acartia clausi.* A second paper (in preparation) deals with the coastal species Calanus marshallae and explores the con- sequences of diel and ontogenetic vertical migration in lengthening the residence time of zooplankton in the upwelling region. The first approach taken to investigate the phenomenon of high inshore zoo- plankton concentrations was to make order of magnitude calculations of the ex- pected cross-shelf transport of those zooplankton whose source is the estuaries, bays and shallows along the coast. It was found that advection predicted from classical upwelling theory (Smith, 1968) with no regard for the time dependency in the winds that force the upwelling circulation, overestimates the extent of offshore transport of these zooplankton. To resolve this discrepancy and to separate the effect of * The systematics of this species are in doubt (Bradford, 1976). <f' IRL Press Limited, I Falconberg Court, London Wl V5FC, U.K. 43 J.S.Wroblewskl biological processes such as copepod development and mortality from the role of advection in determining the spatial structure of the population, a time dependent simulation model was constructed. A numerical upwelling circulation model (Thompson, 1974) predicting the zonal velocity field as a function of wind stress and bottom topography was coupled to a population dynamics model for inshore copepods, specifically Acartia clausi. The biological formulations are discussed in the next section. Published parameter values for A. clausi population dynamics were applied and Downloaded from https://academic.oup.com/plankt/article-abstract/2/1/43/1463810 by Old Dominion University user on 08 July 2019 the biological equations solved first without spatial dependence. A sensitivity analysis was performed to determine the behavior of the model in response to changes in the stage development and mortality parameters. Next the physical dynamics were added. The upwelling circulation model is described here, along with the boundary and initial conditions, and the numerical scheme used for the spatial A. clausi model. Simulations of the distribution of A. clausi are presented for both a theoretical, continuous upwelling case and an actual upwelling period in August 1973 when zooplankton samples were collected. A critique of the model is given in the last section. This paper is a theoretical analysis of the data presented in Peterson et al., (1979). To explain the lack of offshore transport of A. clausi, these authors suggest the existence of an inshore, clockwise rotating gyre, a modification of the two cell, zonal upwelling circulation theory of Mooers, Collins and Smith (1976). Unfortunately there is insufficient current meter data to confirm or deny the exis- tence of such a gyre. It is shown here that this modification is unnecessary if one considers the intermittency of upwelling and field mortality of the animal, both of which act to limit the cross-shelf distribution of A. clausi. Formulation of Acartia clausi dynamics To demonstrate why large inshore concentrations of copepods are not to be ex- pected during upwelling, consider the general equation for the distribution of a nonconservative variable, Z (here, numbers of A. clausi m~3) in the sea -^ + V uZ - V- (K,y Z) = sources and sinks (1) where t is time, u represents the horizontal and vertical water velocities, and K, is the coefficient of the eddy diffusivity in the coordinate direction i. The first term is the local change in Z, the second represents advection of Z, and the third represents turbulent mixing. Sources and sinks refer to the biological processes (e.g. recruitment and mortality) whereby Z becomes nonconservative. Let us choose a Cartesian coordinate system in which y is the longshore direc- tion, x is positive toward the coast and z is positive downward. If we assume a nondivergent flow field and neglect diffusion for the moment, the seaward distribution of Z can be simply expressed as The horizontal velocity u is positive towards shore. Parameter f} describes the rate 44 Model of Acartia clausi daring Oregon Up welling of change in Z with time. If there is a continual source of A. clausi at the coastal boundary to balance the advective and biological losses at sea, the steady state solution to equation (2) can be written where Zois the concentration at the coast. If a mean value of u in the surface layer, time averaged over the two month upwelling season, is - 10 cm sec~'(Bryden, 1978) and the lifetime of the copepod Downloaded from https://academic.oup.com/plankt/article-abstract/2/1/43/1463810 by Old Dominion University user on 08 July 2019 is 60 days (Conover, 1956), adults could be found to the farthest seaward extent of the upwelling zone. However observations (Peterson et al., 1979) show the e-folding length scale (i.e. the distance travelled in reducing Z to Zee"1) to be at most 10 km (Figures 1 and 2). Therefore either the measured value of u is grossly in error (unlikely) or the mortality rate B is significantly higher than death by aging would allow and many animals disappear before their lifespan has elapsed. We can use a numerical cir- culation model to refine our estimate of u. Since the e-folding length scale is fixed from observations, the model gives an indirect estimate of the hard to measure KILOMETERS 0 5 10 IS NAUTICAL MILES ?7 A r FIg.l A chart of the central Oregon coast showing locations of transects sampled during August 14-16, 1973 (from Peterson et al., 1979). 45 J.S.Wroblewikl DISTANCE FROM SHORE (KM) 25 20 15 10 5 I 45# 12' N NESTUCCA LINE Downloaded from https://academic.oup.com/plankt/article-abstract/2/1/43/1463810 by Old Dominion University user on 08 July 2019 14 AUGUST 73 25 20 10 45*06'N . CASCADE HEAD LINE 15 AUGUST 73 25 20 15 44*40'N NEWPORT LINE* 16 AUGUST 73 Fig.2 Spatial structure and abundance (number m~3) of copepodite and adult Acartia clausi sampled during August 14-16, 1973. Dots indicate Clarke-Bumpus net sample depths (from Peterson eta!., 1979). (Prepas and Rigler, 1978) field mortality rate, p. To simulate the offshore transport of Acartia observed during the period of field sampling, August 14-16, 1973, a numerical upwelling circulation model (Thompson, 1974) was used to predict horizontal and vertical velocities as a func- tion of wind stress. The zonal distribution of the four life stages of A. clausi (egg, nauplius, copepodite and adult) off the Oregon coast was modeled by the two dimensional (x, z, t) equation + U w -K* -K. = population dynamics (3) 9t ax where n = 1, 4 such that Z, represents number of eggs m~\ Z2 is nauplii, Z3 is copepodites and Z4 is adult concentration. As A. clausi does not demonstrate any appreciable vertical migration behavior (except perhaps adult females who may vertically migrate over several meters; Landry, 1978; C.B.Miller, personal com- munication), w in equation (3) has no biological component. Note the horizonal and vertical coefficients of eddy diffusivity have been assumed constant. In reality Khand Kvare functions of velocity and mixing length scale. However the flow field employed here to advect the dependent variables has been predicted by a physical model which explicitly incorporates vertical mixing processes into its 46 Modd of Acartia clausi during Oregon Upwdling dynamics. Thus one need not specify the spatial structure in the eddy diffusivity. The temporal variability in the velocity field results in a turbulent flux of Zn which is a property of the flow.