Francisco E. Werner

Regime shifts in marine ecosystems: detection, prediction and management

Regime shifts are abrupt changes between contrasting, persistent states of any complex system. The potential for their prediction in the ocean and possible management depends upon the characteristics of the regime shifts: their drivers (from anthropogenic to natural), scale (from the local to the basin) and potential for management action (from adaptation to mitigation). We present a conceptual framework that will enhance our ability to detect, predict and manage regime shifts in the ocean, illustrating our approach with three well-documented examples: the North Pacific, the North Sea and Caribbean coral reefs. We conclude that the ability to adapt to, or manage, regime shifts depends upon their uniqueness, our understanding of their causes and linkages among ecosystem components and our observational capabilities.

Comprehensive coastal circulation model with application to the Gulf of Maine

Abstract A state-of-the-art finite element model is described and applications are shown for the Gulf of Maine. The model is three-dimensional (hydrostatic) with a free surface, fully nonlinear, incorporates advanced turbulence closure and operates in tidal time. Variable horizontal and vertical resolution are facilitated by the use of unstructured meshes. Solutions for the Gulf of Maine illustrate performance in the context of several isolated nonlinear processes. Composite solutions for March–April and July–August time periods are recorded under climatological forcing. The solutions exhibit a general cyclonic central Gulf circulation, a coastal current with several branch points and anticyclonic circulation around Georges Bank. Each of these features is seasonally modulated. The surface circulation is in general agreement with surface drift observations. The circulation at depth shows the combined influence of deep basin topography and baroclinicity.

Trophodynamic and advective influences on Georges Bank larval cod and haddock

Using a model-based approach, the relative effects of advective and trophodynamic (feeding and growth) processes are considered on populations of larval cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) on Georges Bank. Building on previous studies that describe the role of advection, this study incorporates trophodynamic relationships to examine starvation mortality and growth rates at the level of individual larvae on the Bank. Estimates of prey concentrations and flow fields appropriate for late winter/early spring are used. Both trophodynamic processes and advection influence larval losses from the Bank where, in the absence of starvation, advective losses are on the order of one-fifth of the eggs and larvae spawned on the Bank. Starvation is most important in the first feeding larvae and is much reduced for older larvae. The contact rates between larval fish and zooplankton prey when turbulence is included are 2–5 times greater than the contact rates with no turbulence, and allow the model cod larvae to achieve growth rates similar to those observed on the Bank, although mean rates for larval haddock are still lower than observed. Turbulence-enhanced contact rates are thus determined to be a necessary component in our description of the growth of cod and haddock larvae on Georges Bank. Model cod larvae with growth rates comparable to those observed in the field are located below the surface layer (deeper than 25 m) and inside the 60 m isobath. The region of highest retention due to circulation processes (Werner et al., 1993; Fisheries Oceanography, 2, 43–64) coincides with the region of highest growth rates and highest larval survival. Therefore, there is a complementary interaction between trophodynamic and circulation processes, with those larvae most likely to remain on the Bank also being those in the most favorable feeding regions. Haddock larvae require higher prey densities than cod larvae to survive.

Spatially-explicit individual based modeling of marine populations: A review of the advances in the 1990s

Abstract The utility of individual based models (IBMs) is that properties of ecological systems can be derived by considering the properties of individuals constituting them. Individual differences may be physiological, behavioral or may arise from interactions among individuals. The differences result in unique life histories, which when considered as a whole give rise to growth and size distributions that provide a measure of the state of the population. Early IBMs generally did not consider the effect of a spatially variable physical environment. Recent advances in ocean circulation models that include realistic temporal and spatial variation of currents, turbulence, light, prey, etc., have enabled IBMs to be embedded in model flow fields and for unique, sometimes behaviorally modified, Lagrangian trajectories to be computed. The explicit consideration of realistic spatial heterogeneity provides an additional factor that contributes to the differentiation among individuals, to variances in population structure, and ultimately to our understanding of the recruitment process. This is particularly important in marine environments where fronts, boundary layers, pycnoclines, gyres and other smaller spatial features have been hypothesized to play a significant role in determining vital rates and population structure. In this paper we will review the status of research on spatially-explicit IBMs, their successes, limitations and future developments. Examples will be drawn from approaches used in the past decade in GLOBEC, FOCI, SABRE and other programs.

Modeling zooplankton dynamics

Publisher Summary This chapter discusses the models dealing with marine mesozooplankton, and partially discusses the models dealing with limnic zooplankton, fish larvae, and meroplanktonic larvae. The chapter aims to present detailed, specific applications of models on bioenergetics or demography of zooplankton, rather than a general mathematical study of modeling. The chapter also presents the process models dealing with physiological functions of individual organisms or specific links between physiological parameters and biological functions. The first step in building a model is to be clear on the objectives. These objectives determine the scope of the model, as well as the kind of model to use and the output required from the model. The general method for building a model of a complex system is to identify simpler components of the system and to describe the interactions among these components and external variables of the system and among the components themselves.

The role of hydrodynamics in explaining variability in fish populations

A review of the physical processes present in coastal regions and their effect on pelagic stages of flatfish populations is presented. While quantitative understanding of processes affecting cross-shelf transport and exchange continues to be a fundamental problem shared by physical oceanographers and fisheries scientists studying the early life history of flatfish, advances in hydrodynamic and coupled physical-biological models have made it possible to begin to examine population-level implications of environmental processes. There is now a need to rank these processes in terms of their impact on recruit strength. Existing paradigms provide testable frameworks for explaining the role of physical variability in the observed population patterns, abundance and variability. Identifying explicit links between physical variability and recruitment could result in new approaches to fisheries management strategies.

UPPER-OCEAN TRANSPORT MECHANISMS FROM THE GULF OF MAINE TO GEORGES BANK, WITH IMPLICATIONS FOR CALANUS SUPPLY

Abstract Potential upper-ocean pathways for the supply of biota from the Gulf of Maine to Georges Bank are investigated by numerically tracking particles in realistic 3-d seasonal-mean and tidal flow fields. The flow fields, obtained from a prognostic model forced by observed M2 tides and seasonal-mean wind stress and density fields, include the major known observational features of the circulation regime in winter, spring and summer — a wind-driven surface layer (in winter and early spring) overlying seasonally-evolving baroclinic and tidally-rectified topographic gyres. The surface layer in winter and early spring, with generally southward drift for typical northwesterly wind stress, can act as a conveyor belt for the transport of biota to Georges Bank, provided that the biota can spend a substantial fraction of time in the surface Ekman layer. The numerical experiments indicate that the upper-ocean drift pathways for biota in the southern Gulf of Maine are strongly sensitive to biological and/or physical processes affecting vertical position in relation to the surface Ekman layer and horizontal position in relation to topographic gyres. The seasonality and location of the identified pathways are generally consistent with observed distributional patterns of Calanus finmarchicus based on the 11-year MARMAP surveys.

Field verification of Wave Equation tidal dynamics in the English Channel and southern North Sea

Abstract Finite element results obtained with the implicit Wave Equation algorithm are compared with field data in a blind (uncalibrated) verification exercise. The study area comprises the English Channel and the southern North Sea.

A comparison of two finite element models of tidal hydrodynamics using a North Sea data set

Abstract Using the region of the English Channel and the southern bight of the North Sea, we systematically compare the results of two independent finite element models of tidal hydrodynamics. The model intercomparison provides a means for increasing our understanding of the relevant physical processes in the region in question as well as a means for the evaluation of certain algorithmic procedures of the two models.

Circulation, mixing, and exchange processes in the vicinity of tidal inlets: A numerical study

The circulation in the vicinity of an idealized tidal inlet connecting a continental shelf and a coastal sound is examined. The circulation is forced by an M2 tide and a weakly buoyant discharge. The buoyant discharge forms a plume in the coastal ocean and induces a distinct anticyclonic circulation at the plume edge that is maintained throughout the tidal cycle. We focus on the plumes onset and its evolution over 5–10 tidal cycles. Over the timescales considered, the plume was roughly circular, slightly skewed in the along-coast direction. The model solution yielded high vertical Ekman numbers Eϵ ∼ O(5) in the vicinity of the inlet mouth, decreasing seaward from the inlet to an order of magnitude smaller (Eϵ ∼ 0.25) at the seaward edge of the plume. Passive particles released in the region seaward of the inlet mouth were used to describe the exchange between the coastal region and the inlet. A marked asymmetry between ebb and flood flows is observed in the vicinity of the inlet, with jet-like ebbing currents and weaker potential-flow-like flooding currents. Over a tidal cycle, net exchanges between the coastal ocean and the inlet are found to be spatially and temporally dependent; that is, particle trajectories depend on the release point and the time of the release in the tidal cycle. The near-inlet residual circulation shows significant differences in the absence of stratification.