The terms that we use in diving are often quite baffling to the uninitiated. I once had a student in an introductory (resort) course express concern that we were going to make a “two tank dive” because she was certain that she’d “never be able to carry two tanks on her back.” Another example is the term “boat diving.” It does not mean that we intend to dive on a boat; that’s called wreck diving. “Boat” merely describes the way we get there. Similarly, when we talk about shore diving, all we mean is that it’s a dive where the entry is made from shore (as opposed to a boat). In fact, “shore” is merely something that we pass through on our way to deeper water. Little, if any, attention is ever paid to the interface between land and sea, and the zone immediately beneath it. After all, what on earth could be of interest in this shallow and easily accessible margin?
In reality, this transitional zone from the terrestrial world to the marine realm can be one of the most productive and diverse ecosystems that you’ll ever visit. Here is where some organisms hide when they’re young. Yet, ironically, it’s often also the place where only the hearty can survive. It’s a region of paradox — of ebb and flow — where some of the most interesting observations and relationships in the ocean can be found. For those who’ll take the time to appreciate it, the shallows near shore can be a magical place.
You’ve Got To Be Kidding
As a scuba diver, you may ask, why on God’s green earth I would care to waste my time in the shallows near shore. Aside from the interesting residents and interactions that take place there — which we’ll learn about in a minute — the reasons are numerous. Sometimes during bad weather, diving close to shore is the only alternative to not diving at all. I have on many occasions been “blown out” on a boat dive, and rather than retreat disappointedly back home, managed to salvage memorable experiences exploring a protected shoreline. Furthermore, diving in this sheltered shallow environment — often with less than ideal visibility — encourages a leisurely and attentive pace conducive to observing tiny creatures in their intricate relationships with one another. It’s also the environment where I first began to appreciate something to which I’d never paid attention: marine algae and plants.
But there’s still another advantage to diving near shore that cannot be matched in any other marine environment. That advantage is accessibility. This is where we can introduce nondivers to the place that means so much to us. A guided trek through a tide pool is often an excellent way to hook a nondiver into taking the next step: snorkeling. This, too, is a benefit. Snorkeling is the preferred method of experiencing this shallow aquatic world. Freed from the weighty burden of scuba tanks, and the frightening cacophony of its bubbles, snorkelers are able to approach shore zone residents as if they were part of the aquatic family. And snorkeling makes shore diving a less expensive and less threatening experiment for the not-so-sure-I-want-to-be-a diver’s first experience.
Diving in the shore zone is also a good remedy for some reluctant scuba divers. I learned this many years ago while teaching scuba divers off the coast of Southern California. For many of my more timid students, spending an idyllic day in only a few feet of water snorkeling on the beautiful rocky shores off Laguna Beach gave them the courage to progress to diving in deeper water. Without this confidence builder, many would never have completed certification.
From a broader environmental perspective, exploring the shore has still another benefit. Nowhere is humankind’s degrading effect on the marine environment more evident than in the shore zone. Therefore, if we’re interested in truly being ambassadors for the environment, we can make no greater contribution than opening the eyes of our friends, dive buddies or students to this delicate and vital realm — and what we’re doing to it. However, appreciating and conveying the unusual nature of the shore requires a basic understanding of its ecology. As most diving takes place on tropical coral reefs, an appropriate place to begin our trip along the shore is with the ecosystem that defines tropical coastlines: mangrove forests.
Only recently have we begun to understand the vital role of wetlands. For centuries these forbidding and often stinking coastal margins were considered wastelands. Most assumed that they were worthless at best, and at worst, the site of pestilence and disease. Even the term malaria — literally “bad air” — derived from the belief that the disease was caused by the putrid air of tropical wetlands. In tropical regions of the world, the ubiquitous emergent marsh grasses seen in temperate latitudes are replaced by a group of unique and hearty trees able to withstand the onslaught of salt water. This generally unrelated group is referred to collectively as mangroves. (A term derived from the Portuguese word for tree: mangue, and the English word for a stand of trees: grove.)
As mangrove forests appear to be one of the least inviting places on earth, few divers would ever consider them a suitable snorkeling site. And that’s a shame. Under the right conditions, a snorkel tour through a mangrove forest — particularly for macro and close-up photographers — can be the highlight of a tropical holiday.
I became acquainted with mangrove snorkeling many years ago while taking groups to the Florida Keys. During the unpredictable weather of winter and spring, it’s not terribly uncommon for the winds to howl and keep every dive boat in the archipelago securely tied to the dock. One year, in desperation to salvage a weather-ravaged trip, I took a group snorkeling through a mangrove forest in north Key Largo. I, along with the intrepid few who accompanied me, were utterly amazed by the experience. In that single dive we witnessed four-inch barracuda, finger-sized lobster and spectacular invertebrates that I’ve still not been able to identify. I was so taken by the experience that, regardless of weather, I’ve repeated my mangrove excursions on almost every subsequent trip. Of course, my disbelief at the wonders that abound in a mangrove forest was due to an utter lack of knowledge of — and outright revulsion to — this unique ecosystem. (For a vivid but vicarious tour of mangrove forests, I encourage you to see the wonderful new book “Mangroves: Trees in the Sea by Jerry & Idaz Greenberg,” Seahawk Press.)
While it’s impossible to discuss mangrove ecosystems in any comprehensive sense in a limited article, there are some basics about their ecology that you should know. Once covering nearly 75 percent of all tropical and subtropical shorelines (they’re now disappearing faster than tropical rain forests), mangroves include roughly 50 species worldwide. However, only three species predominate in Florida and much of the Caribbean. Normally occurring closest to open ocean, and thus able to best tolerate the intertidal zone, is the red mangrove (Rhizophora mangle). It’s easily identified by its intricate and eerie system of reddish prop roots. The black mangrove (Avicennia germinans), sometimes intermingled within the red but usually further inshore, has cablelike roots and numerous breathing tubes (pneumatophores) that are exposed at low tide. The white mangrove, (Laguncularia racemosa) with its gray-white trunk and flattened oval leaves, occurs in the driest region, often above the mean high tide mark. As there’s little oxygen available in the mucky soils where mangroves grow, all species use breathing tubes or pores to draw oxygen into the root system. Within the muddy bottom of the mangrove forest live specialized anaerobic bacteria that break down compounds such as sulfate ions (SO4) to capture their oxygen for metabolism. The liberated sulfur then combines readily with the plentiful hydrogen atoms in the mud, forming gas with the unforgettable smell of rotten eggs — hydrogen sulfide (H2S). Because of the less-than-pleasant smell of mangroves, many assume that they’re polluted. Not so. In fact, the smell is an indication that everything is working the way it should. (If you’re turned off by the smell, then avoid standing in the mud and disturbing the bottom.)
Mangroves typically occur in three regions. They’re most obvious along shorelines where they fringe bays and lagoons. As they do near my home, mangroves also occur in riverine environments where tidal surge moves up rivers and creeks, producing wide variations in salinity. Third, mangroves are seen on — or comprising entirely — low islands where they are periodically overwashed by flooding tides.
Mangrove forests are dependent upon the leaves that continually rain down in amazing quantities — about three tons per acre per year. But rather than being washed away by the tide, this leaf litter is trapped by the mangroves’ entangling root system. There the leaves are broken down by various invertebrates, bacteria and fungi. These decaying leaves become the base of an entire detrital (decaying organic matter) food web. The food web is supplemented by other plant material, guano from birds roosting in the trees, and organic matter of all kinds including dead animals and loose seagrass trapped in the maze of roots.
The detritus is fed upon by a variety of organisms such as amphipods, fiddler crabs, and fish such as killifish and minnows. They in turn are eaten by larger animals until, finally, still larger predators enter the fray. Many of the resident organisms can withstand wide variations in temperature and salinity, and are found in few other locations.
Mangroves also serve as vital nursery grounds. For example, I’ve seen almost every fish that occurs on the coral reef as juveniles in the shallows of mangrove forests. They are the primary breeding ground for many of the world’s shrimp species. And like shrimp, lobster larvae floating in the plankton migrate to the roots of red mangroves for both food and protection. (Baby lobsters are commonly seen in mangrove roots.) Even small sharks are found in the channels and roots of the mangrove forests.
These hearty trees perform even more vital functions. Along many tropical shores, their massive density and secure rooting protects the coastline from severe storm damage. Working in the opposite direction, mangroves trap storm water or other shore-based effluent and, like all wetlands, process pollutants and retain high levels of nutrients. Unfortunately, when mangrove forests are eliminated for development, this “waste water treatment” function is eliminated, and the consequences are disastrous for nearby ecosystems such as coral reefs. Finally, over time, the leaf litter and buildup of soil in the root system contribute to the creation of new land. In fact, Native Americans of Florida and the Caribbean, describing this land-building function, called mangroves “the walking trees.”
Life at Shore’s End
Of course, not all diving takes place in the tropics. In fact, it’s often near shore where coldwater environments can compete with the explosion of life found in tropical waters. Although intertidal zones (the area wet at high tide and dry at low) occur in tropical waters, the minimal tidal range normally confines the rocky shore environment there to a relatively small and virtually unnoticed area. (For a good introduction to tropical rocky shore, see “Peterson’s Field Guide to Southeastern and Caribbean Shores,” published by Houghton Mifflin Co.) In temperate regions, however, this is often very different. Here, the intertidal zone is often expansive and comprises one of the most interesting marine environments. Especially in rocky coastal regions with significant tidal range, intertidal communities can be expansive habitats, critical to the local marine and terrestrial ecology. What’s more, these are also great places to introduce nondiving friends to the wonders of the marine realm, often without even getting wet.
Because they’re so easily accessible, much of what we know about marine ecology has been gleaned from intertidal shores. In fact, one of the first scientists to view the marine environment as an “ecosystem” studied the intertidal zone of the central California coast. His name is Ed Ricketts, and he was a close friend of the author John Steinbeck. (They were such good friends that Steinbeck used Ricketts as a model for the character “Doc” in several of his novels.) As a result of this long history of research, our knowledge of the ecology of rocky shores is one of the most complete views that science has of any marine habitat.
Perhaps the most striking characteristic of all rocky shores is that the plants and animals that live here tend to group in distinctive patterns, with some living high in the intertidal zone and others at lower tidal levels. This is called “zonation,” and the abundance and distribution of species composition varies according to factors such as tidal range and exposure to wave action.
While we rarely think of it on those clear and calm days when we visit tide pools, the intertidal zone is really one of Earth’s harshest environments for life. Think about it. Organisms that live here must be able to withstand conditions of daily inundation by the sea, alternating with a cycle of exposure to terrestrial conditions. This has driven intertidal animals and plants to evolve numerous adaptations to resist desiccation (drying out) at low tide. These adaptations include the ability to tightly clamp to the surface of rocks, as do limpets; closing their shells with a tight-fitting operculum, as seen in snails; or retreating to tide pools, crevices, burrows or beneath seaweed to avoid the drying action of heat and wind. Conversely, as the tide returns, these organisms must be able to withstand the battering of waves. Although they lack true roots, marine algae (seaweeds) have developed tenacious structures called holdfasts that attach securely to rocks. Even some animals have similar structures. For example, the holdfasts of one of the intertidal’s most common residents — the mussel — are structures called byssus threads. Oysters and barnacles take a different tack and attach themselves directly to the surface of rocks with cement that’s so hard dentists are studying it to learn how to make better fillings. Remaining in place with the return of the tide isn’t the only challenge. At low tide, intertidal zones are prime hunting grounds for seabirds and even mammals. When the tide returns, they must once again contend with predation from marine critters. Talk about a no-win situation.
Zonation begins at the “splash zone” or, more properly termed, the supratidal zone. This includes the area wetted only during the highest of spring tides or by breaking waves. Its inhabitants include tiny snails called nerites, some crabs, and an insectlike crustacean known as a sea roach (Ligia oceanica). If soil can accumulate in cracks and crevices, even a few hardy, salt-tolerant plants can be found.
Next is the littoral — or more properly the midlittoral — zone. This is the largest zone on a rocky shore and extends between the high and low tide marks. In the upper regions of this zone are found periwinkles in what can only be described as extraordinary densities; up to 10,000 snails per square meter. A little lower, barnacles take over and form a sharply demarcated band. Next, is the “mussel zone,” which is limited by the fact that they cannot tolerate exposure to air as well as their barnacle cousins can. Crowding together like a New York street during rush hour creates intense competition, and helps define important ecological relationships that we are only now beginning to fully understand. Below the midlittoral is the subtidal zone — below the lowest low tide — where marine conditions are constant.
Algae also follow a pattern of zonation. Within the upper splash zone are often found encrusting black lichen (a combination of fungi and bacteria) and blue-green algae (cyanobacteria). A little lower, yellow, white and gray lichens are found. In the upper intertidal regions, green seaweeds are common. Below the green seaweed zone one encounters the brown algae, including the common rockweed (Fucus sp.), with its characteristic gas-filled bladders. Below the brown seaweed zone, red seaweeds occur, and finally, in many temperate regions, the subtidal is the realm of the largest brown algae, kelps, such as Laminaria, Nereocystis and Macrocystis.
But just as living in a city has its advantages, so too does such crowding of the intertidal zone. Like neighborhoods, these tight-knit groups create microhabitats that retain moisture when the tide is out, thus preventing desiccation. Crowding also increases the chance of finding mates when it comes time to reproduce. For example, although barnacles are hermaphrodites, they do not self-fertilize. And given that they cannot move to find a mate, lots of close-by neighbors are an absolute requirement for survival.
Tidal range is not the only factor that determines zonation. Other physical and biological causes enter the picture. The upper limit is often controlled by an organism’s ability to deal with exposure to air and wide fluctuations in temperature and salinity. Biological factors come into play as well, such as the absence of suitable food, or through grazing pressure from terrestrial animals (especially seabirds).
Both attached benthic (bottom) algae and phytoplankton are important primary producers supporting rocky intertidal communities; and this is not an easy environment in which to survive. But here temperate shorelines have an advantage. For example, in tropical intertidal zones, benthic algae must survive heavy rainfall, high light intensities and exposure to high air temperatures with the resulting desiccation. Conversely, in polar regions, freezing and scouring by ice limit algal production. So, it’s in the temperate region where benthic algae reach their full potential.
As with the animal residents, physical and biological factors also determine zonation patterns among benthic algae. Yet there’s an added complication: plants must compete not only for restricted space but for sunlight, too. The upper zonation limits of algae are determined by their tolerance for exposure and desiccation. As with animals, grazing pressure is also an important factor. And even humans can affect the formula.
Just as grazing animals like wildebeest and gazelles help to control the ecology of the Serengeti, attached algae in the intertidal zone are grazed by organisms such as limpets, chitons and sea urchins. In turn, plankton nourish the filter-feeding mussels, barnacles, clams, tunicates, sponges and tubeworms. And just as the Serengeti has its lions, the intertidal also has its predators. In this case, the lions are the ravenous starfish, which eat limpets, snails, barnacles, mussels and oysters. There are also many species of predatory snails that eat a variety of prey such as barnacles, mussels and clams. Among important predators are sea anemones that are one of the few sessile species to prey on shrimp and even small fish. The intertidal also has it scavengers — the vultures and hyenas of the Serengeti — including various isopods and crabs. And unlike in other deeper-water marine environments, even shore birds are significant intertidal predators.
Predation also has other effects on the ecology of the intertidal zone. For example, experiments have documented that predators such as urchins, limpets, chitons and some snails control both the level of primary production and the species composition of benthic algae. Experiments have demonstrated that by removing limpets, their lack of grazing results in both increased algal production and a change in species composition. The same has been shown when sea urchins are removed from the community.
Another example of the role competition and predation plays in species composition and diversity can be seen along the northwest Pacific Coast of North America. Here, the rocky intertidal zone is dominated by mussels, barnacles and the carnivorous starfish, (Pisaster ochraceus). When this starfish is removed from the community, the result is a lowering of species diversity from about 30 to a single species of mussel. But when it’s present, Pisaster’s feeding controls the numbers of the dominant mussel, thereby keeping space open and preventing any single organism from taking over. Pisaster is called a keystone species because its activities, as one ecologist has noted, “disproportionately affect the patterns of species occurrence distribution and density in the community in which it lives.” A similar situation occurs in the rocky intertidal zone of New England. Here a different species of mussel would completely dominate the community were it not kept in check by certain starfish (Asterias sp.) and a snail (Nucellus lapillus).
As you can see, shorelines are not just gateways to the sea. They contain communities that are vital to nearby marine and terrestrial ecosystems. They offer a fascinating and easily accessible glimpse into the science of ecology. And they are places where both divers and nondivers can learn more about how the ocean functions, and often provide graphic evidence of humankind’s abuse of our planet. So, the next time you go diving, don’t ignore the shore.
Hints on Exploring the Shore
1. Plan your dive for high, slack tide (when current is minimal). Outgoing tides typically carry with them high levels of sediment, and thus poor visibility. Incoming tides tend to carry clearer water. High tide also makes snorkeling more interesting since a greater area of the intertidal is covered, allowing you to see more critters.
2. While divers are often admonished not to wear gloves to protect delicate organisms such as coral reefs, this is one place where you should definitely opt for hand protection. Rocky shores are usually covered with barnacles and other organisms that can cause cuts and scrapes. Mangrove roots are sometimes covered with hydroids and other stinging organisms. (Still, avoid intentionally touching or handling organisms.)
3. When diving on rocky coasts, be aware of changing surf conditions. A glass-flat day can, with little warning, become a maelstrom.
4. When diving in mangrove forests, be careful to maintain your orientation. The channels and inlets within mangroves can be like a maze, and it’s easy to become lost. If you’re diving in an area frequented by boat traffic, be sure to carry a diver-down flag and stay out of the channel. Boaters do not expect to find snorkelers in mangrove forests.
5. Because you’ll be maneuvering in shallow water around structures or organisms that can scrape or sting, always wear some form of full-exposure protection. Depending upon local conditions, anything from a 3-mil wet suit to a thin skin suit is recommended.
6. When snorkeling with nondivers, always have some form of flotation such as an inner tube or surface float in case they require assistance or need to rest.