CLASSIFIERS OR MODIFIERS FOR AQUATIC ECOSYSTEMS

Although the UNESCO classification system is usually considered to predominantly cover terrestrial formations, it does include vegetated aquatic ecosystems. Within formation classes I-VI terms such as "flooded," "riparian," and "waterlogged," are used to describe ecosystems that are wet or covered with water on a periodic or temporary basis, or even constantly in the case of certain palustrine formations. These ecosystems include bogs, flushes, salt marshes, flood savannahs, sedge swamps, and numerous other water dominated ecosystems.

In addition, formation class VII, Aquatic Plant Formations, encompasses systems in which water covers the land constantly or most of the year. This formation class includes five formation subclasses. Each of these subclasses has a distinct assemblage or set of species that usually occupy different niches of an aquatic ecosystem depending on water clarity, depth, flow velocity, etc. Several formations may occur within a short distance of each other, and in many cases they are not mappable at a scale of 1:250,000 as used in Central America. The Map of the Ecosystems of Central America project reviewed a variety of existing classification systems (including Salm and Clark 1984, Gómez 1984 1986c; Green et al. 2000) but finally they determined that the original UNESCO system categories were adequate to describe aquatic ecosystems with a distinguishable vegetation cover above or under the water surface. The recognised distinct vegetated aquatic classes all have distinct floristic species assemblages. The variation of differentiation of aquatic faunal assemblages may be more determined by some of the physical characteristics. The USNVC and LCCS offer equally suitable options.

A considerable challenge is the classification of aquatic ecosystems. The botanists that thought out the physiologic ecological classification systems, primarily were terrestrial botanists, with great knowledge about terrestrial vegetation. Many parts of water bodies lack any vegetation at all while characterization and classification without protruding vegetation is severely hampered by the extremely limited reflection of sunlight of both water and submerged elements.

In order to reach analytical completeness to deal with all biodiversity the UNESCO classification system and their derived systems need an additional class to classify aquatic ecosystems with little or no vegetation cover: "Open Water ", which mapping team for Central America added as class VIII. These are predominantly covered by water and have less than 10 percent of their area covered by emergent or submerged vegetation. Such class is also needed for the USNVC. The LCCS has a few open water classes, but the system needs more subdivision. The aquatic component of each system needs more elaboration, but with customised identification, sufficient distinction can be established to classify aquatic ecosystems with distinct species assemblages.

One of the primary reasons why terrestrial ecosystems can be classified and mapped conveniently by using the vegetation as the main proxy for ecosystem characterization is that the largest biomass is accumulated in – sessile – plants. In aquatic ecosystems, the flora primarily consists of planktonic algae with a relatively low biomass but an extremely high turnover. The largest biomass is primarily accumulated in planktonic and mobile fauna. None of these elements is sufficiently static to be observed as a group, the composition is often highly dynamic and there is no morphological structure. To add to this problem, remote sensors are very restricted in their underwater visibility. Data often must be acquired from direct measurement, underwater observation and sampling, all costly and slow methods.

Moreover, it should be realised that aquatic ecosystems are often subject to considerably seasonal changes. Their chemical composition and physical appearance vary over short periods and thanks to the mobility of many aquatic species, many aquatic ecosystems show a great seasonality in the species compositions.

In order to determine how aquatic ecosystems can be distinguished in such a way that partially different species assemblies can be separated, one must analyse what the essential sets may look like and which classifiers or modifiers can tell them apart.

When present, aquatic vegetation protruding through the water surface are distinctly determining classifiers or modifiers for the identification of aquatic ecosystems. As the protruding part are directly exposed to the atmosphere, one may call these "atmospheric aquatic vegetations". In this sense, floating vegetation should also be considered atmospheric aquatic vegetation.

Rooted atmospheric aquatic vegetation may: indicate stagnant or relatively slowly flowing water. They provide distinct habitat for a variety of plant and animal species that without the vegetation structure would not be there. Many species use vegetation to hide from predators.

Surface water systems without atmospheric aquatic vegetation have been dubbed “Open Water Systems”. The other principle groups of organisms characteristic of aquatic ecosystems are:

1. plankton;
2. benthos;
3. mobility of organisms.

Plankton

The composition of plankton is highly volatile and may change in hours due to weather conditions, such as light exposure, wind, etc, as well as to changing currents. As such, plankton is not a good indicator of an ecosystem but a number of classifiers or modifiers are highly influential on plankton composition. Usually they include a combination of physical and chemical modifiers of the water phase, such as:

Many of these classifiers or modifiers are volatile in their consistency, sometimes difficult to assess, highly variable over short distances with continuously changing distribution. As a result, for classifying planktonic species assemblies, most physical and chemical conditions of the water phase should be used with great reservation.

To further the division of open water systems for Central America, it was determined that salinity was the most important divisive characteristic. Most marine species are separated from limnic (freshwater) species by higher concentrations of salt. Among them primary freshwater fish species are stenohaline, meaning that they have little tolerance for variations in salinity. Secondary freshwater fish species have a varying tolerance to salinity. Some may tolerate temporary exposure to undiluted seawater but it may not be their preference. Other species are adapted to switching back and forth between saline and freshwater systems and may do so with some frequency or during some phases of their life cycle. Most marine species will never enter freshwaters and are also stenohaline. Species in brackish waters are euryhaline, meaning they are tolerant of significant fluctuations in variations in salinity. The ictiofaunal assemblages for limnic, brackish, and marine systems are partially distinct and the degree of salinity is considered the single most distinctive factor for aquatic ecosystems, which – besides differentiation in atmospheric vegetation components - is particularly clear in the composition of the partially different assemblages of fish species.

Primary indicators for biotic differentiation a new classification, the following division would be necessary from a point of view of protected areas informatics:

Like for terrestrial ecosystems, when present, vegetation is a determinative component for aquatic ecosystems, creating habitat for significant assemblages of species that could not be present without vegetation. Wooded swamps usually fall under Formations I, V, or VII. Lakes and rivers often have fringes of emerged vegetation that are classified under formations V or VII in the UNESCO system. Limnic open water systems lack major areas of aquatic vegetation that would allow their classification under the UNESCO system. Within freshwater ecosystems some useful modifiers are:

Nutrient level: divided into eutrophic, mesotrophic and oligotrophic. Usually oligotrophic systems are relatively poor in species which are highly specialised. The water may be dark due to colorants released from anaerobe decaying processes of plants. Transparency may nevertheless be reasonable. Mesotrophic ecosystems are the most species diverse freshwater systems. Transparency may be high, but this is not always the case. Eutrophic systems may occur under natural conditions, particularly in coastal lowlands where clayy soils abound, but increasingly eutrophic conditions are the result of pollution loads. Natural eutrophic ecosystems have characteristic species, often resilient to temporary low oxygen contents and they have relatively low transparency during the growing season. Pollution related eutrophic conditions at moderate levels may be similar to those of natural eutrophic systems, but at high nutrient levels the species composition becomes very low, oxygen levels may approach zero and the water becomes very non-transparent and green from algae bloom.

The pH level: Most water systems fluctuate close to neutral conditions. Oligotrophic systems in areas with peat formation may have pH levels as low as less than 4. As pH values are easy to measure in the field, low pH values may be used to indicate oligotrophic water conditions. Some species need some dissolved calcium carbonate (Meyers 1966). The latter can be interpreted from the general characteristics of the soil if this is a calcareous region.

Current velocity: Some species require rather quiet water with slow flowing to stagnant waters or "lentic" systems, while others require the conditions of strong currents, "lotic" systems. Whether it is the current in itself or conditions that come along with them, like abundance of exposed rocks and/or gravel beds, high oxygenation, lower temperatures associated with streams in the upper watersheds in not clear. Current velocities may be deducted from terrain accidentation, drainage characteristics and the shapes of the water bodies. Wide rivers usually have at least some lentic systems, while lakes and ponds may have a current flowing through them.

Geomorphological shapes and location of the water systems: The Map of the Ecosystems of Central America distinguishes a variety of lake types based on their geological origin. There is no good evidence however, that the geological origin contributes much to the distinction of species assemblies, and from a protected areas informatics point of view the distinction is not required.

Rivers can be devided up in upper stream, mid stream and low land river systems, although the whereabouts of this division is rather subjective. Upper and mid stream rivers are in principle freshwater systems – unless upstream highly saline waters or salt deposits exist - populated with primary and secondary freshwater fish species. Mid stream rivers have the highest diversity in freshwater fish species. There is a rather steep decrease of fish species as elevation increases. Particularly along the Andes, this difference is dramatic. With an extraordinary diversity of fish species until about 600 masl, the diversity decreases rapidly and above about 1,000 masl only some 20 – 30 representatives can be found along the entire Andean region, all being members of just one single genus, Astroblepus spp., in Spanish know under the name "preñadillas". These are abundant non-migratory fishes that live in highly turbulent waters. In Central America, the appears to be somewhat lower, at 300 and 600 m respectively or wherever the currents become too fast to allow for many species to survive.

At least close to the coast, lowland rivers often have fluctuating levels of salinity and they may be predominantly populated with secondary freshwater, facultative freshwater and peripheral fish species. Upper streams usually have rocky or gravely bottoms, while in lowland rivers the bottoms usually consist of sediments each with a distinct benthos.

Geological barriers are important classifiers or modifiers for determining the composition of assemblages of fish species. Most endemic fish species are in geologically fully isolated locations, like remnant wells in deserts or small isolated watersheds. Probably all endemic freshwater fishes are known to science and their locations are known. There is no need to further identify them as isolated ecosystems, like endemic cichlids in the Fauna and Flora Reserve Cuatro Cienegas and Cyprinodon spp. in Death Valley National Monument. A divisive factor in assemblies of fish species are the water systems. While the ecological conditions may be the same, the species assemblies may vary among watersheds. In the watersheds of Central America, a gradual shift in species can be observed from North-west to South-east. A baseline would need to assess if watersheds with distinct fish populations are present and if the distinguishing fish species be represented. The watersheds of large rivers like the Amazon and the Paraná rivers need to split up as many species only occur in parts of the watersheds.

The shapes of the water systems are probably irrelevant, but they may be proxys for certain conditions, like current velocity, salinity, composition of the water bottom, etc. Usually, it is not necessary to identify all upperstream creeks, as they will be automatically incorporated in any protected areas system covering all ecosystems and 10% of the territory of a country, while quite a few streams will originate in each protected area system. Midstream and lowland rivers will usually merely pass through a protected areas. It is necessary to identify and map them as parts of the protected areas baseline. For most countries fairly detailed baselines exist for fish species, that allow the prediction of their occurrence in watersheds and for large rivers like the Amazon and Paraná in parts of the watersheds. For many protected areas this may be rather indicative for the potential assemblage of fish species.

Estuarine open waters are very distinct from both marine and limnic water systems, as they are characterized by varying salinity and usually very high dynamics. Estuaries – the coastal waters (river mouths and deltas, lakes with permanent or temporary outlets to the sea, barrier enclosed coastal seas, etc.) where fresh water and sea water mix - often have high sedimentation, low transparency, high algae content and low species diversity, but high organic productivity, highly specialized organisms and high fauna population densities.

In many tropical estuarine tidal zones, mangroves abound, while in all climates bare tidal mud flats and periodically inundated salt marshes may be found, all of which can be identified with the physiognomic ecological ecosystem classification systems.

A distinction was made on the Map of the Ecosystems of Central America between semi-closed and open estuaries. In retrospect, however, there probably is no clear ecological reason for maintaining this distinction, as the probably all organisms that characterize those waters occur in both of them. The primary distinctions from a protected areas informatics perspective are:

Marine ecosystems are areas that are below the low tidal line and permanently under water. Photosynthesis is one of the key factors determining the species composition of marine ecosystems, which among other things depends on:

Photosynthesis first increases with temperature, then reaches a maximum, above which it the photosynthesis starts to decrease again (Kondratyev et al 2004). With regard to light penetration this varies greatly depending on the location of the earth. In the pelagic or open ocean realm, light may penetrate to a depth of 400 - 500 m but along the coasts this is much less depending on the abundance of turbidity caused by sediment particles and plankton. The crystal-clear waters allow sunlight to penetrate considerably deeper than around coral reefs, where the water is often teeming with plankton. In tropical coastal waters, photosynthesis may take place until around 150 m. Coral reefs are built by stony corals that harbor in their tissues symbiotic algae called zooxanthellae. These algae need sunlight for photosynthesis, which they can only perform until about 60 m mentioned for the Pacific Ocean in Hawaii (Bishop Museum 2008) and 50 meters in the Atlantic ocean (Leite Prates et al 2003). Thus the reef-building stony corals dominate only those reefs shallower than 50 – 60 m. This depth is also the maximum depth to which scuba conventional scuba diving is possible. This zone is also referred to as the permanently submerged littoral zone.

Beyond that, our knowledge of the oceans is significantly less. In the zone between 50 and 150 m, light penetration is very limited, the water temperature is rather low and the composition of the species assemblages change, making way for soft-bodied marine organisms, such as sponges, soft-corals, tunicates, and other sessile invertebrates (Bishop Museum 2008). It therefore seems logical for the tropics to use the 50 or 60 m isobaths and the 150 m isobaths for separating the permanently submerged littoral ecosystem and a relatively shallow pelagic system and gradually less towards higher latitudes.

Another important zone is the tidal zone in which organisms live that can survive periodic exposure to the air and being submerged. Humby and Harborne (1999) provide a very detailed classification system for coral reefs in the Atlantic Ocean. It can be downloaded from http://www.birdlist.org/downloads/ecology/coral_classification_scheme.pdf. This method however depends on mapping by scuba divers and it is too costly to produce maps at this level of detail.

As one moves away from the equator, light penetration may decrease as light enters more under an angle. Thus the delimitation of the littoral marine zone is lower in depth, as may be the case in waters with permanently lower transparency.

For most coasts bathymetric maps exist, but they may not always coincide with the preferred isobats of separation. In absence of solid bathyometric models, it is preferable to use the nearest isobats on the map for ecosystem identification. For availability of bathymetric maps see http://www.ngdc.noaa.gov/mgg/bathymetry/maps/nos_intro.html.

As the term is traditionally used, littoral systems also encompass tidal zones, which may include beaches, salt marshes, and mangroves, and even coastal plains; all identifiable with physiognomic ecological classification systems. Within the permanently submerged littoral zone, sea floors may be rocky, silty, sandy, or gravely. In particular, areas of sea grass can be classified as submerged vegetation classes. Sessile marine macro-algae often occur among corals (although in coverage, they usually are much less important than corals), and at times, may be important enough to be mapped as submerged marine fixed macro-algae. In coralline areas such cover often indicates disturbance. This may be of human or origin or caused by native conditions such as hurricanes (for example near Punto Allen in Sian Ka’an Biosphere Reserve, México) or prolonged periods of excessive heat (Gúzman, pers. com). In general, marine ecosystems are much less varied than terrestrial ecosystems, which is particularly due to extremely good connectivity and much smaller climatic (smaller temperature variations and much more limited precipitation influence) variety in aquatic ecosystems. The greatest variability in marine ecosystems increases from deep to shallow and from soft versus hard substrates.

Pelagic systems have not yet developed into a major issue in protected areas informatics, as thus far, protected areas have been bound within the 200 miles economical zones of nations. Most pelagic systems within marine protected areas have been chosen for the protection of very specific fauna (particularly whales) and are within the 50 – 150 m isobats. Thus far, only 3 mappable classifiers have come to mind: depth, bottom accidentation (including ship wrecks) and hard versus soft substrate. These conditions can be identified systematically with ship-based sensors. The ARGO project has been measuring water seawater temperature and salinity between the surface and a depth of 2000 m in all oceans of the earth since 1990, using large numbers of "Argo floats" or buoys. These measurements may be instrumental in helping determine more pelagic ecosystem differentiation over time. Information about the study and all freely data are available at http://www.argo.ucsd.edu.

Mobile organisms

In aquatic ecosystems, most lower organisms for their distribution primarily depend on their distribution on water currents. Many benthic species pass from a planktonic phase to a benthic phase in the course of their life cycles, in which they often can be distinctly observed during their benthic phase, while remaining rather elusive during their planktonic phase. Also actively swimming organisms like crustaceans and a variety of fish species pass through a planktonic phase during their life cycles, while a variety of mobile species associate themselves with specific conditions of the water bottom (coral reef fish species, bottom feeding fish species, etc.), making the bottom characteristics an important modifier for such species.

Vreugdenhil et al 2002 suggests that fishes may be valuable indicators for distinguishing between some aquatic ecosystems. After further consideration, this would not seem to be a good idea. Meyers 1966 describes how he sampled many water systems in Central America and often would not come up with more than 3 or 4 of the same fish species. Characterizing an aquatic ecosystem through such laborious sampling is too costly and too slow. The combination of a few physical classifiers or modifiers and salinity samples will be more efficient.

A few words should be dedicated to water and shore birds and marine mammals and turtles. These taxa are rather selective in their choice of habitat. While it is possible to assess their preferred habitats by a rather limited suite of physical, chemical and vegetation conditions, their actual presence is by no means guaranteed by seemingly suitable conditions like a tidal mud flat rich in benthos, waters rich in fish, etc. Other factors are involved, like disturbance and the simple habit of a species to go to a certain location of preference. Therefore, aquatic ecosystems must be overlaid with maps of congregations of fauna elements.

Benthic organisms

The composition of bottom characteristics separates distinct species assemblages: Some benthic fauna and flora can only live in soft bottoms, while other species live on top of the bottom and require a hard substrate for their attachment (e.g. corals). Many mobile fauna species (e.g. Cod, coral fish species) prefer to stay near hard objects like boulders, submerged rockscapes, shipwrecks, etc. particularly if they provide hiding places for escape. Several Salmoids need gravel beds for spawning. Salm and Clark (1984) provide several bottom modifiers that may be used to characterize open water formations and the composition of such formations can have very characteristic species assemblies (e.g. Corals, benthic communities on mud flats, benthic communities in gravel beds in creeks, etc. Mumby and Harborne (1999) provide detailed classes for coralline coasts, but at that level of detail, not all coralline classes reflect distinct assemblages of species (Guzmán 1998). While bottom composition is extremely important, it is believed that a limited number of well-chosen

For aquatic ecosystems distinctions should be made for: