The importance of lagoon fisheries worldwide, especially to developing countries, prompted this effort to investigate the links between the hydrodynamics of lagoons and their productivities. Besides the fact that there are good theoretical reasons to expect such relationships, lagoons are highly amenable to hydraulic manipulation. If such relationships could be established, the inherent productivity of a lagoon might be enhanced with present day ocean engineering technology.
Furthermore, if a lagoon were beneficially altered hydraulically, for example by changing the dimensions of a pass, it could result in continued increased production with little additional investment - in contrast to supplementing nutrients or stocks, for example, which must be continually maintained. Hydraulic manipulations to increase natural productivity, in a sense, represent passive methods of further exploiting the natural productivity of lagoons.
Lagoons are a class of estuaries, at least as variable in form as any class of aquatic environment, linked by the common characteristic of having a single (or more) restricted connection(s) to the ocean. Many estuarine classification schemes (Pritchard 1955, Hansen and Rattray 1966, Caspers 1967, Pritchard 1969, Odum and Copeland 1972, Lankford 1976, Fairbridge 1980, Day 1981, et al.) include lagoons, but none seems adequate, since the debate continues (Hedgepeth 1983) on the merits of different schemes. Some of this debate is understandable, reflecting different regional perspectives and uses for the schemes. Unfortunately, none of the schemes to date has emphasized quantifiable characteristics for which researchers could collect data which would enable ordination of lagoons along any but the coarsest scales.
Recent attempts at classification (e.g. Kjerfve 1986) tend to focus on more dvnamic aspects, however, even these are still fundamenta -y non-quantitative and simplistic. In any case, it is unlikely that a biologically relevant classification system consisting of any 2 or 3 categories can be developed for lagoons. Given the accepted definition of an estuary (Pritchard 1967), which includes the requirement of a “free connection with the ocean”, the fact of restricted pass exchange necessitates a special category for lagoons. General discussions of lagoons are found in Colombo (1977), Barnes (1980) and Hedgepeth (1983).
It is difficult to envision much progress in synthesis and understanding of differences and similarities among estuaries until attention is focused on collecting more comparable biologically and hydrodynamically relevant data. A protocol for this is desperately needed, and could be encouraged by a classification scheme emphasizing quantifiable dynamic gradients (Hansen and Rattray 1966) instead of broad categories.
For the purposes of this manual, we will adopt the crude definition of a coastal lagoon as an estuary with a restricted connection (pass or passes) to the ocean such that wind forcing of currents is more important than astronomical tidal forcing. A further characteristic of many lagoons is the orientation of their principal topographic major (and minor) axes parallel (and perpendicular) to the coastline. (Note: this definition has eliminated the coral reef “lagoons” which are open embayments often separated from the ocean by a submerged reef.)
Equally variable as form, but on average high, is the productivity of lagoons (Ben-Tuvia 1983, Ardizzone 1984, Kapetsky 1984). Fisheries yield may range from about 2 to over 800kg/ha/y and is 10-20 times higher per unit primary production in lagoons than lakes (Nixon 1982), suggesting either a greater conversion efficiency or greater harvest efficiency, or both. The low diversity of species and the relatively shallow water in lagoons could contribute to either. Much of the variability in yield among lagoons is due to varying intensity of harvest (Kapetsky 1984), but the thesis of this manual is that a large fraction of this variability is attributable to differences in hydrodynamics. Hydrodynamics may directly affect catchability (Corsi and Ardizzone 1985, Chauvet 1988), but there are many ways in which the hydrodynamics shape the production dynamics of lagoon systems.
While most lagoon fisheries biologists are aware of the importance of hydrodynamic processes in coastal environments, their publications do not reflect the fact that virtually all biological processes in coastal environments are strongly affected, if not controlled, by physical forcing. Moreover, most investigations do not include either the hydrographic or the current, wind and water level measurements necessary to assess the degree to which the hydraulics may have determined the biological outcome.
Thus, it seems desirable to review the physical/biological relationships that have been established, and to suggest other relationships which, if quantified, would likely improve our ability to predict and manage the fisheries production of lagoons.
The ideal way to establish such relationships would be an empirical analysis of “before and after modification” cases. Unfortunately, while many lagoons have been hydraulically altered, either intentionally or unintentionally, there are few cases where the biological processes or conditions were determined before and after modification. There are even fewer cases where the hydraulic response was adequately described - and virtually no cases where both the physics and the biology were synoptically studied on similar scales. Therefore, an “experimental” approach was precluded. The best alternative was to extract such relationships post facto by examining the present biological status of lagoons in relation to existing hydrodynamic conditions. There have been some attempts to establish such relationships (e.g., Deegan et al. 1986), but with limited success. Because many of the hydraulic factors are autocorrelated, a large data set is required to assess the importance of each. Furthermore, there is the uncertainty of the relationship between production and yield, and the latter is most often the only information available. Inferences will always be less convincing than relationships derived from experimental approaches, but may achieve a useful level of precision for management. A final way to gain insight is to examine the relationships between hydrodynamics and production within lagoons large enough to contain a spectrum of hydrodynamic conditions. This approach suffers from the obvious possibility that inferences from one lagoon cannot be transferred to another. Our perspective is that all such methods of investigation can help to establish at least qualitative relationships between hydraulics and production. Once identified, these relationships should be subjected to experimental quantitative verification. Only then will we have the necessary predictive capability for hydraulic manipulations.
As a final perspective, it is critical to understand the factors limiting fisheries or aquaculture production in any particular lagoon before hydraulic modification is attempted. Ecologically, lagoons may be expected to differ in two important ways from typical wide-mouthed estuaries. The production in some lagoons may be limited by the number of colonizing stages owing to their relatively poor connection to the coastal ocean. Such lagoons are inherently capable of more production in their present states, if colonization can be enhanced. In such cases, hydraulic modification to increase production must be aimed at increasing colonization. In lagoons with relatively wide, or multiple passes, the carrying capacity may be typically reached (or exceeded) by the numbers of colonizing stages. This is the more familiar ecological paradigm. Hydraulic modifications of these lagoons must be aimed at increasing the carrying capacity. In the first case, increases in carrying capacity (e.g., improving water quality), while perhaps increasing the growth or survival rate slightly, will not result in substantial gains. In the second case, where the carrying capacity is already reached (or exceeded), hydraulic manipulations which increase the number of colonizers may even depress the production, for example, by increasing competition for limiting resources.
A second difference lies in the fact that water residence time in lagoons is typically much longer than in other estuaries. This leads to the expectations of different responses to nutrient additions, greater standing stocks of planktonic organisms, a greater risk of stagnation, and, in general, a tighter coupling of abiotic scalars to meterological conditions. Limnological principles may be more relevant than those of marine systems.
A distinction must be made at the outset between hydrologic and hydraulic (or hydrodynamic) models. Hydrologic models predict mean water residence times, water budgets, and the like - necessary for assessment of nutrient supply. In a sense, an hydrologic model is a 1-cell black-box with inputs from the ocean, inlet stream (s) and atmosphere. But, while water residence time may provide a clue to the effect of nutrient loading on the trophic state of a lagoon, hydraulic models which predict flow patterns are needed to address most of the questions of biological interest in lagoons. The data demands of an hydraulic model are considerably more sophisticated than the typical hydrologic model requires. Hydraulic models provide the information necessary to understand how the production dynamics might vary within the cell (lagoon).
Hydraulic models can fall into fifteen classes, since there are three spatial dimensions and time as independent variables. Permutations of 1-, 2- and 3-dimensional steady state and time-dependent models can exist. For coastal lagoons, hydraulic models generally fall into four classes, 2-dimensional (vertically integrated) and 3-dimensional, either steady or unsteady. All of these latter models vary spatially in size of “cells”, or spatial resolution. In general, the arguments for adequacy of a 2D model depends on a lack of vertical stratification, but as Pietrafesa et al. (1986) have shown, even with unstratified conditions, 2D models can underestimate water movements to a considerable degree. For example, steady state vertically averaged flow is zero. However, in small, well-mixed lagoons, some questions concerning mean flow can be addressed using 2D models (Millet 1989). The scale of resolution required depends on the questions of biological interest and the morphometry of the lagoon, but in general, vertical spatial scales of 10's to 100's of centimeters, horizontal spatial scales of 100's to 1000's of meters, and temporal scales of hours to days are required. As Millet (1989) shows, sediment size distribution, if accumulated over long periods, can be correlated with annual (or longer) time scale hydraulics, but plankton abundance is transitory. To underscore a previous point however, even the most primitive 3D hydraulic model is better than the typical hydrologic model for most questions of biological interest, as will be seen later.
Conceptually, lagoons are viewed as shallow and well-mixed and consequently are modeled as soup dishes, minimizing complexity and providing the generic physics. However, the biology of lagoons has been found to be local, so if the biology of lagoons is to be coupled to the physics, then the physics must also be spatially local. Thus, while the simplicity of the typical hydrologic model and of vertically averaged models is appealing, topographic influence is of major importance in most lagoons. If physics is to be used to understand the biology, then the physics must be modeled over scales equal to, or less than, the biology.
The objectives of this manual are four-fold:
to provide a synopsis of the biologically relevant physical oceanographic (hydrodynamic) processes in lagoons for fisheries biologists and managers (Chapter 2);
to review the relationships between physical forcing and fisheries production in lagoons (Chapter 3);
to point out where hydraulic manipulation of lagoons has increased, or might be expected to increase, fisheries yield (Chapter 4); and,
to outline a protocol for the hydrodynamic research necessary to improve our capacity to predict or manipulate the yields of lagoons through fishing or aquaculture (Chapter 5).
Note: The technical depth of the chapters varies. Readers interested in management of lagoons may wish to skip Chapter 2 (Physical processes in lagoons), or read only the chapter summary.
