Having outlined the principal hydraulic features of lagoons and how they are related to production, we now consider how lagoons might be hydraulically manipulated to enhance productivity. Application of any such principles to a particular lagoon, of course, must be predicated on a choice of target organisms and thus, the desired environment of that system (Caddy and Sharp 1986). Ideally, the limiting factors would have been identified in order to maximize the benefit/cost ratio of manipulations. An hydraulic model of the lagoon is necessary to predict the effects of manipulations on most of the important biological processes.
Most attempts at hydraulic manipulations of lagoons have been for the purpose of correcting rather obvious problems. In some cases, manipulations resulted in additional problems; this attests to the present lack of understanding of how lagoons function hydraulically and how hydraulics are linked to production. Even in the few cases where lagoons were purposely manipulated, before and after studies of quantitative links between the physics and biology were not performed, at least to the point where we can predict, with confidence, the outcome of a similar manipulation in another system. Therefore, such predictions (and this chapter) must necessarily be based on generally untested principles.
Before the biological effects of any hydraulic manipulation can be predicted, certain basic information needs to be available on the lagoon's biota. For example:
Is primary production in the lagoon nutrient-limited? Light-limited?
Is the carrying capacity of target species typically reached?
Is the system driven by planktonic or benthic primary producers?
What are the abiotic optima for the target species?
What are the water quality characteristics of freshwater inputs?
Is the flushing time of the lagoon greater or less than the generation time of producers? Consumers?
There are three principal manipulable hydraulic features of lagoons: 1) input of freshwater; 2) exchange with the ocean; and, 3) the internal circulation. All of these are linked hydrodynamically, so that modification of any will modify all.
Input of freshwater can be altered by diversions of water to or from existing input streams and by installing gates or dams on these streams. Both the quantity and schedule (if storage capacity exists) of releases can be manipulated. Freshwater input may be modified for several reasons: 1) to control salinity; 2) to divert pollutants; or, 3) to control nutrient additions.
Increasing freshwater input into a lagoon will have four principal physical effects. First, mean salinity will be lowered. Second, mean flushing rate will be increased. Third, the lagoon's tendency to stratify, both vertically and horizontally, will be increased. Fourth, more seawater input will ensue, particularly if vertical stratification is enhanced. In general, increased freshwater input will cause lagoon hydraulics to move toward those of drowned river valley estuaries, particularly in the vicinity of the inlet streams.
Unless large volumes of freshwater enter the lagoon differing markedly in quality, including pollutants, increasing freshwater input will generally have little immediate effect on the overall abiotic environment of most lagoons. This is partly because increasing freshwater input reduces the mean water residence time, and partly because the abiotic environment (temperature and oxygen) is coupled so tightly to the atmosphere. On the other hand, cumulative effects, of nutrients for example, may be substantial. Or, if enough freshwater is introduced so as to cause the lagoon to stratify vertically, then the hydraulic characteristics of the lagoon can be altered, with widespread effects. Frequently, inlet streams are sources of inorganic turbidity. Increasing freshwater input may decrease light penetration and increase thermal stratification, especially in the vicinity of inlet streams.
The most intense effects will occur in the area(s) of the lagoon receiving input(s). Temperature variation will increase, at least in most temperate lagoons because most freshwater input streams are warmer in summer and cooler in winter. Mean salinity will decline, but the primary effect will be to create a more extensive salt wedge, and to further isolate bottom waters from the atmospheric influences.
Very little is known about the nearshore physical processes that advect offshore larval stages to passes. Longshore currents are often strong in the near coastal ocean, and their variation is likely to be a major determinant of the entry of larvae (Miller 1988). Increased freshwater input will have little effect on the numbers of fish reaching the pass, unless they are actively migrating and cued by odors, etc.. If larvae are cued by odors, the lower salinity of the lagoon, or other “estuarine water factors”, then increasing freshwater input to the lagoon may have considerably greater impacts on the colonization process (Gandolfi et al. 1984, Miller 1988).
Although the flood volume is probably the most important determinant of the numbers of larvae entering a lagoon through a pass, the effects on fish entering the pass depend on the degree of stratification and the volume of freshwater input. If the water in the pass is stratified, increased freshwater input, by increasing the ebb volume, will cause increased entry of sea water, potentially increasing colonization. If flow through the pass is turbulent, as is probably most often the case, increased freshwater input may decrease the average flood volume, potentially reducing passive colonization. The effect on retention of larvae also depends on stratification, but in general, increased freshwater input will reduce the fraction of new seawater entering on a flood tide which remains in the lagoon. In general, increasing freshwater input will reduce colonization, at least by passively advected stages.
Where two passes exist, and one functions primarily as a flood channel and the other an ebb channel, increasing freshwater input is much more likely to increase colonization by increasing the likelihood that flooding sea water entering the flood channel will be retained in the lagoon (Yanez-Arancibia et al. 1982).
The colonization of a lagoon in the vicinity of inlet stream(s) may, however, be substantially altered with additional freshwater input. Increased freshwater input will cause a salt wedge type of circulation, increasing the movement of lagoon water upstream in bottom layers into the embayment. Likewise the movement of surface layers will increase in the downstream direction and away from the embayment. In general, we expect to find more demersal forms and fewer pelagic forms in the vicinities of inlet streams - again, unless the pelagic forms are actively migrating and are attracted by characteristics of the inlet stream water.
With few exceptions, increasing freshwater input will negatively impact planktonic primary production in the immediate vicinity of the inlet stream owing to the increased local flushing rate. Other direct effects of increased freshwater input on primary production depend mainly on the water quality and temperature of the input. But usually turbidity will increase which may decrease primary productivity. The residence time of biota in the upper layers, at least, will be decreased. In hypereutrophic lagoons this is a positive effect. But the tendency for lagoon stratification will increase (Dronkers and Zimmerman 1982), which may isolate bottom waters, and, if organically rich, may induce anoxic conditions. Benthic primary production may be depressed as well, but, unlike phytoplankton, attached algae or macrophytes will not be flushed from the system more quickly. While the above effects may occur in the vicinity of the inlet stream, other, sometimes opposite, effects may occur elsewhere in the lagoon.
The overall response of the lagoon to increased freshwater input will be joint response to the increased flushing rate, lower salinity, and (probably) increased vertical stratification, turbidity and nutrients. The response depends upon the nature of the primary producers. In general, we expect a local reduction reduction in phytoplankton productivity and a shift toward more benthic primary production with increased low turbidity freshwater input. Elsewhere in the lagoon, increases in overall primary productivity usually accompany increased freshwater input because of additional nutrient loading (Kwei 1977, Flint 1985). The overall response can be negative, however, if nutrients are not already limiting.
Perhaps the main positive effect of increased freshwater input will be to expand the region of low and varying salinity, thus potentially decreasing predation by marine predators.
Another positive effect of increased freshwater input lies in the greater tendency for seawater to move into and upstream in the lagoon. This may result in improved colonization by demersal forms. The effect will certainly occur in the vicinity of the inlet stream where a more pronounced salt wedge will form. Furthermore, species which migrate actively in response to odor cues, for example, should increase.
The potential negative effects of increased freshwater discharge on secondary production include a decreased residence time for zooplankton, and possibly decreased benthic secondary production by depressing oxygen there. If a shift toward benthic primary production occurs, a shift toward benthic secondary production may be expected.
In sum, the principle hydrodynamic effects of modifying freshwater input are local, and by increasing the tendency to stratify, tend to flush surface organisms and entrain more demersal forms. The principle hydrologic effects are to increase the ebb volume and reduce the mean residence time. Other effects upon production are related to the water quality of the influent.
Diversion of freshwaters to hypersaline lagoons to alleviate acute stress or to render such systems inhabitable (Okuda 1965, Posewitz 1968, Sanchez 1980, Soberon-Chavez et al. 1986, et al.) have obvious value as hydrologic manipulations. In such cases, fisheries were created where there were none. Hydraulic optimization of existing fisheries is more difficult to justify, because the additional fisheries production cannot be predicted, and, therefore, assessed in advance.
A canal connecting the Rio Presidio to the Huizache Lagoon on the west coast of Mexico was dug in 1977. Although no hydraulic or hydrologic data are given, Edwards (1978) suggested the penaeid shrimp catch may have increased 40% as a result of the canal. No mechanism(s) were determined. The lagoon was apparently subject to “drying out” (Edwards 1978), so the increase may have been proportional to the surface area increase in the lagoon.
In Lake Fogliano, Italy, freshwater input (from a river connected to the sea) was reduced to reduce nutrient loading. Fish yield declined from about 250 to 150 kg/ha/y (Ardizzone 1984), apparently because the larval source (via the river) was reduced. The dystrophic problems were alleviated, however.
Exchange with the ocean can be altered by changing dimensions of existing passes by dredging, filling and jettying. And additional passes can be created. Gates or diversions are necessary to differentially affect ebb and flood flows in the case of a single pass. Depending upon the distance between passes, and the lagoon's orientation with respect to the prevailing wind direction, creation of an additional pass can cause one to function primarily as a flood pass and another as an ebb pass. The local water residence time will decrease in the lagoon between passes. In this case, different sizes of passes can also effect differential flooding and ebbing. Again, depending upon the existing degree of stratification within the lagoon, a pass channel deep enough to allow stratification to persist during flood and ebb currents may allow freer ingress of seawater at the bottom, and freer egress of fresher water at the surface. Thus, in addition to increasing the total exchange, a differential effect upon the lagoon (surface) water and the ocean (bottom) water may accompany stratification in the pass. Most often, ebb and flood flows through a pass are turbulent, and significantly enlarging a pass may encourage stratification.
Since river input becomes a decreasing fraction of the ebb flow as the exchange volume increases, a net increase in mean salinity will accompany any enlargement of a pass (except in hypersaline lagoons). The only way to prevent a salinity increase is to simultaneously increase freshwater inflow. In general this will cause the lagoon circulation to converge upon that of the typical drowned river valley estuary, at least in the vicinity of the inlet stream. At least increased likelihood of stratification will occur. Higher exchange volumes will usually tend to stabilize temperature in the lagoon, since the ocean temperature is usually less variable than that of the lagoon. At temperate latitudes, ocean temperature is usually warmer in winter and cooler in summer than the lagoon, leading to marked thermoclines where wind mixing is low. The temperature of a lagoon often varies more in the vicinities of passes and inlets. The usual effect of pass enlargement is to reduce pollution (in the lagoon only) according to the decrease in mean water residence time, since the source of most pollution is inlet streams.
The net effect of increased exchange depends on the degree to which the greater amount of sea water (and passively migrating larvae) entering is trapped in the lagoon. More larvae will enter, but more larvae may leave. Enlarging passes may reduce turbulence in the pass, which has been shown to increase the efficiency of retention of larvae (van der Veer 1986). In general the effect of increasing exchange will be to increase the numbers of larvae arriving at the pass, entering the pass and being retained in the lagoon. Except in very small lagoons, the effect of pass enlargement on distribution, once in the lagoon will be minimal, since this depends more on the residual (not tidal) circulation.
In the case of multiple passes which function more or less as ebb or flood channels, enlarging either will increase the net flow through the flood pass. Since there is less resistance to ebb flow than flood flow, enlarging the flood channel may be more effective.
In principle, there are other means of increasing the probability of retention. Gates or diversions of flooding water away from the pass, once inside the lagoon, should increase the likelihood of larval retention.
In a few oligotrophic lagoons, additional nutrient loading may accompany pass enlargement, but usually the lagoon is already richer in nutrients than the coastal ocean, therefore the net effect is to decrease nutrient loading (by dilution). In the case of vertically stratified lagoons, enlarging a pass may have even greater effects on nutrient loss, since the surface layers are differentially affected. The main effect in such cases would be to export surface plankton and nutrients at a more rapid rate, decreasing the standing crop. The light regime will be improved, however, since ocean water is usually less turbid than most lagoons. Thus, depending upon the lagoon, primary production may be either increased or decreased with pass enlargement.
Because mean flushing time will be decreased, the mean standing crop of plankton should be decreased. The effect could be a decrease in secondary production. But if secondary production is limited by colonization, or if plankton generation rates exceed the existing flushing rate, pass enlargement may increase secondary production. But while entry of larvae and juveniles will be facilitated, so will that of marine predators. Furthermore, the abiotic effects (stabilized temperature, increased salinity, reduced turbidity) all would tend to favor colonization by marine species. For these reasons, it is not clear whether increased exchange with the ocean would have a net positive or negative effect on secondary production. It seems more likely to be negative.
The response of any lagoon to pass enlargement (or decrease) clearly depends upon many factors which are likely to vary among lagoons. The examples below are illustrative.
Increasing exchange with the ocean may be done for several reasons: 1) to facilitate ingress and egress of biota; 2) to alter salinity; 3) to promote internal circulation; or, 4) to reduce (or increase) nutrients.
Lake Monaci, in the Latium region of Italy, now has one of the highest yields (284–528 kg/ha/y) for a lagoon. Its former dystrophic problems (Ardizzone 1984) were reduced by pumping seawater into the lagoon and periodically opening the pass to the sea. The fish yield increased to its present level, however, a fry stocking program was initiated at the same time, so the relative contributions of the 3 measures are unclear. In contrast, Fondi Lagoon yields 50–150 kg/ha/y, being a deeper lagoon which is permanently stratified and anoxic in the deeper layers. Fondi is connected to the sea by two passes, so the “natural” exchange is better. Hydraulic comparison of these two lagoons with others in the Latium region of Italy would be extremely useful.
Elsewhere in Italy, the valli of the northern Adriatic represent some of the most intensively managed systems in the world. Some of the lagoons are equipped with water scoops to recharge and recirculate water (Ardizzone et al. 1988). Sluice gates are also used to raise the water level, or alternatively, to allow lagoon water to escape to encourage colonization. In winter, a thin sheet of ice is formed by slowly introducing freshwater. This insulates the lagoons from atmospheric cooling, but must essentially stop internal circulation. Although these practices, and others, have evolved over decades of culture, there is no quantitative hydraulic basis to determine the effectiveness, in terms of increased production, of any of the individual practices, except to say they cumulatively prevent certain dystrophic conditions.
The valli would present an excellent experimental system to study, both hydraulically and biologically. The individual lagoons, however, are relatively small, so the wind-induced circulation must be less than in larger lagoons.
Also in Italy, the pass from the Scardovari Lagoon to the ocean is periodically dredged to prevent anoxic dystrophy (Rossi et al. 1984). Although dredging appears to achieve the desired effect, an adequate hydraulic description is lacking.
The lac du Tunis (Tunisia) has been intensively hydraulically manipulated over the years, as well as having been the subject of excellent biological studies (Chauvet 1984, 1986). The system is stressed by both thermal and sewage effluents, yet the manipulations have been successful to the extent that a fishery (primarily for mullet) exists. But, owing to the complexity, and lack of detailed hydraulic investigation accompanying modification, it is difficult to extract a quantative benefit of any specific manipulation. The wind blows rather persistently, but weakly, and Chauvet (1984) recognizes that this may be a key factor in this (and other) system's productivity. Quignard (1984) regards the circulation as rather poor, but the passes have reduced the average salinity of this hyperhaline system, making it more productive.
Salses-Leucat Lagoon in France illustrates the importance of ocean connections and the risks of poorly planned attempts to reduce the mean water residence time by enlarging or creating passes. Quignard (1984) provides a summary of the events. This lagoon formerly was connected to the sea by an intermittent pass. This pass was enlarged and two others created, which caused the salinity to rise from about 10–20ppt to over 30 (Quignard 1984). The fish production fell from about 40 kg/ha/y to less than 10, whereupon bordigues were installed to prevent loss of fish and, presumably, to exclude marine predators.
The fish production in Lake Bardawil, Egypt, was enhanced by dredging the one pass and creating two others (Quignard 1984). Salinity fell from 70–120ppt to around 50. Fish production increased over 2-fold in response.
Olsen and Lee (1982) describe the effects of creating permanent passes to the ocean in three Rhode Island, USA, lagoons. Formerly, the barrier islands were only seasonally breached. The results of pass stabilization were nearly uniformly disastrous in terms of lost fishery production. These cases were somewhat unique in that extensive flats, which were formerly inundated in winter before spring rains caused breaching, were now exposed to freezing temperatures, resulting in a loss of shellfish. However, the examples do point out the need for careful hydraulic studies. A second problem that was exacerbated was sedimentation in the lagoons, which has now created areas of poor circulation leading to anoxia.
The Swartvlei lagoon (S. Africa), like many others, is seasonally open and closed to the sea. Kok and Whitfield (1986) found lower numbers of fish in the lagoon during the closed season. Rather than dystrophic problems, which are typically associated with poor exchange with the ocean, they attributed the lower numbers of fish to the inhibited colonization. If a lagoon pass is closed for long periods of time, e.g., the Bot lagoon (S. Africa), continued freshwater input may decrease salinity enough to cause fish kills (Whitfield et al. 1981).
The principal effects of augmenting the internal circulation within a lagoon are to: 1) increase the vertical mixing (decrease the likelihood of stratification and increase the likelihood of resuspension of particles); 2) decrease the fraction of the lagoon with long flushing times; 3) increase the likelihood of retention of ocean flood water and passively advected larval and juvenile organisms. All of these would seem to have positive effects upon production, unless they are accompanied by resuspensions of large amounts of sediment.
Three methods exist: 1) increase water momentum by enlarging passes; 2) divert the horizontal water movement to problem areas with ditches or channels; and, 3) increase vertical mixing with baffles or spoils.
The effects of pass enlargement on the residual circulation of lagoons were discussed earlier (Section 4.2). The most significant effects on residual circulation would accompany creation of additional passes. Increasing vertical mixing has not been purposely attempted to increase vertical mixing, as it has in lakes and ponds (Boyd 1979), but baffles or spoil shoals in lagoons could make use of the momentum to divert water vertically. This is a passive solution, requiring only the initial installation costs, providing the material is not eroded. Owing to the large quantity of kinetic energy in any moving water mass, a relatively weak horizontal current would be sufficient to displace water vertically. But the effect would be quite local. There are a few cases where ditches or channels have been used to direct water towards “dead” areas of small lagoons. In all cases, predicting (or assessing) the effects of any modification requires an hydrodynamic model, and these do not exist for most lagoons.
The principle effects of augmenting circulation on the abiotic environment are local or indirect. However, these effects can be substantial, if for example, by increasing contact with the atmosphere or bottom, exchanges are facilitated. The principle negative effects are associated with increased suspension of sediments.
A major effect of augmenting internal circulation is to advect flood waters away from the pass, reducing the likelihood that the same water will leave on the next ebb. This results in favorable conditions for larval retention. The effect on larval entry is negligible, but larvae can be hydraulically directed to areas formerly not colonized.
The principal effect of flow augmentation or redirection is to locally decrease mean residence time and increase vertical mixing. The effects on primary production were discussed in Section 3.3. This can be used as a tool to alleviate local problems associated with stagnation, oxygen depletion, pollution, etc. There is the risk of increased turbidity from sediment sources, but if this does not occur, benthic primary productivity will be encouraged. Most of the effects on primary productivity are positive.
Besides alleviating problems associated with stagnation and redirecting colonizing stages to an area, the effects of flow augmentation on secondary production of pelagic organisms will be positive up to the point where high turbidity occurs. Benthic organisms may benefit from advection of additional planktonic food to an area.
Canals or ditches have been dredged in, or between, lagoons to facilitate transport of water to “dead” areas (Olsen and Lee 1982). The source of energy is tidal, and the benefits must be weighed against the (usual) accompanying increase in salinity. Since these solutions represent pervasive changes of the lagoon environment, pervasive changes in the ecosystem should be expected. Such projects have, while successfully eliminating local problems, resulted in others on a much larger scale (Olsen and Lee 1982, Crawford 1984).
Another hydraulic solution to the problem of low oxygen is to increase vertical mixing. Increasing vertical mixing replaces lower layers with relatively (usually) oxygen-rich upper layers and recharges the upper layers by facilitating diffusion of oxygen at the air-water interface. An estimate of the energy required to mix the water column can be obtained from the relationship between wind speed and depth of mixing. This might be a temporary solution in localized shallow areas, as it is in lakes.
The problem of cold stress in winter is alleviated in Northern Adriatic lagoons with a thin surface layer of freshwater which is allowed to freeze (Ardizzone et al. 1988), thus effectively reducing contact with cold air (equivalent to reducing vertical mixing). The principal effect is insulation, but some “greenhouse effect” may also occur.
We have outlined principles in this manual which must be selectively applied to different lagoons. There is no single “hydraulic prescription” for all lagoons. For instance, if the primary productivity of a lagoon is light-limited, then increasing freshwater input may depress production; if nutrient limited, it may increase it. Or if insufficient larvae typically colonize the lagoon, increasing pass dimensions would be expected to increase production; if not, the same manipulation may only invite additional marine predators, which may depress production.
There are few cases where increases or decreases in production have been quantitatively linked to hydraulic manipulations - especially mechanistically. This is attributable to the lack of “before and after” data on either production or hydrodynamics. Until such data are available for a variety of cases, hydrodynamic manipulation will be an “experiment”. Although it is almost certain that productivity can be enhanced by hydrodynamic manipulations in most lagoons, to date most manipulations have been attempted to alleviate obvious or acute problems in stressed lagoons. Kapetsky (1981) lists several of these. Until a lagoon is adequately described hydrodynamically as well as biologically, manipulations should be considered “blunt instruments of repair” in most cases. Alleviating one problem may create another.
Although the principal forcing function, wind, in lagoons cannot be manipulated, the internal circulation can be redirected, both vertically and horizontally, by altering the bottom topography. Redirecting necessarily results in a loss of momentum, therefore the mean current velocity in the lagoon will decrease proportionally. For example, if the effect of a shoal is to increase vertical mixing, the kinetic energy required will be that necessary to change the center of gravity (potential energy) of the lagoon. Such manipulations should be considered as tools to alleviate local problems, since the effect of a single barrier will be quite local, and the amount of energy necessary to substantially change the center of gravity of the entire lagoon is enormous. Effectively, this is a redistribution of energy. What is gained locally will be lost elsewhere. And unfortunately, generally when additional mixing is most necessary (during windless periods) the least kinetic energy exists in the system. Solar energy should be considered as a possibility to augment local vertical mixing. A 1 cm2 of lagoon surface on a cloudless day in the tropics receives enough energy to lift about 105 cm3 of water about a meter, depending upon density differences, et al.. And, where persistant winds blow, this source of energy should be considered as a means of redirecting water movements. Lunar tidal energy in a lagoon can be increased by increasing the inlet dimensions or creating new inlets, but in lagoons, lunar tidal energy is limited. Either shoals or suspended baffles placed perpendicular to the flow near the pass could have differential effects on surface and bottom waters, especially if stratified flow conditions exist. Directional gates could certainly be installed and differentially affect the flood or ebb flow.
Our last chapter outlines the necessary research protocol to put hydrodynamic manipulation of lagoons into a more scientifically enlightened perspective. Meanwhile, biologists and hydrodynamicists should attempt to capitalize on “natural experiments”, such as the seasonal or storm-related changes in pass dimensions to begin to determine the fundamental relationship between hydrodynamics and production.
