Solitary seagrass meadow in Chile supports a unique ... - Bedim

Seagrasses are found in shallow waters in protectedbays and estuaries throughout the world. The 60described species of these flowering plants (Angiosperms)are completely adapted to the marine environments. Thespecies that forms the seagrass meadow from PuertoAldea, Heterozostera tasmanica, is one of them. Thisspecies is native to the southern coast of Australia, whereit forms extensive meadows. The only other place outsideof Australia from where H. tasmanica has been reported isin northern Chile (Figure. 1). How did this species reachthis distant place? The wide extension of the meadow inPuerto Aldea suggests that H. tasmanica has probablycolonized the Chilean coast a long time ago. The fact thatanother smaller meadow has been reported about 400 kmnorth of Puerto Aldea is even more puzzling because, sofar, no flowering plants of H. tasmanica have been foundin Chile. Consequently, one must assume that propagationis primarily vegetative. During the past few years, rumorsabout other isolated patches have increased, suggestingthat H. tasmanica may be successfully dispersing andexpanding its range along the northern Chilean coast.While the dispersal mechanism and populationdynamics of H. tasmanica in Chile is of great interest tothe marine scientist, the fishermen of Puerto Aldea aremore interested in the ecological interactions developingunder the seagrass canopy because this meadow shelterstheir principal resource: the scallop. Since a few years thefishermen from Puerto Aldea care for this natural gardenof scallops, under the concept of participativemanagement. They have associated themselves and theymonitor ‘their’ scallop stock regularly, and they only exploitwhen scallops are sufficient in number and size. The closeassociation between the seagrass meadow and thescallops facilitates this gardening. Becoming aware of theimportance of the seagrass meadow has awakened theinterest among fishermen to learn more about theecological relationship between the seagrass and ‘their’scallops.FUNCTIONS AND BIODIVERSITYOF THE SEAGRASS MEADOWSeagrass meadows provide ahabitat due to their structuralcomplexity and stabilizing properties.They supply refuge and food formany organisms that live among theseagrass matrix, both plants andanimals. This is immediately visibleto the fishermen of Puerto Aldeawho encounter large abundances ofinvertebrates and fishes in ‘their’seagrass meadow. Scallops(Argopecten purpuratus) andcommercially exploited crabs(Cancer setosus) are among themost common large organismsfound in the seagrass meadow ofPuerto Aldea. When opening thedense cover of the seagrass canopyother creatures come into sight,clumsy juveniles of crabs try to crawlaway, dense aggregations of shrimpdart into all directions, and severalspecies of colourful sea anemonescatch the attention of the interesteddiver. Further down, among theseagrass rhizomes, tube-dwellingpolychaetes and filter-feedingsponges abound. Dense schools ofjuvenile fish hover above theseagrass meadow, occasionallydescending down between theshoots to prey on the abundantfauna living in the meadow. To date,117 different organisms have beenreported from the seagrass meadowJMBA Global Marine Environment 2

FIGURE 2. The seastar (Meyenaster gelatinosus),one of the important predators of scallops inthe seagrass meadow of Puerto Aldea.of Puerto Aldea (Gonzalez, 1990), but mostlikely the diversity is even higher, becausethere are some species that could not yet beidentified.The fishermen from Puerto Aldea haverecognized one of the principal ecologicalfeatures of ‘their’ seagrass meadow – it servesas an ideal nursery ground for the species,which they exploit commercially. They easilysee the large numbers of juvenile crabs andscallops, and follow their development day byday. The dense aggregations of scallops alsoshow them that their seagrass meadow is avery productive place, and they perceive thatthe seagrass provides abundant detritus totrophic webs. Being out in their seagrassmeadow year-round they also notice that manypredators are attracted to the seagrass bed –fishes, crabs and seastars. The fishermen haveidentified the principal enemy: the voraciousseastar Meyenaster gelatinosus (Figure. 2).They see the seastars preying on theirscallops. Perceiving the threat that seastarspose to their resource, the fishermen have, onseveral occasions, attempted to take control byactively removing seastars from the seagrassmeadow. Since the local scallop garden is awell defined area in a dense seagrass meadowthis approach appears feasible and fishermenhave gone one step further to becoming theactive care-takers of their herd of scallops.However, while it is true that seastars prey onscallops, their prey spectrum is not limited tothese tasty bivalves. Seastars also feed on awide variety of other organisms present in theseagrass. Among these additional prey speciesthrive several species of herbivorous snails.Thus, removal of seastars might releasepredation pressure on these grazers, therebyindirectly affecting the seagrass itself.Abundant populations of snails might lead to areduction of seagrass cover and thereby adeterioration of the living conditions for thescallops. The fishermen are collaborating withecologists from local universities in order tolearn more about the delicate ecologicalbalances in their seagrass meadow. They knowthat good care-taking requires a goodunderstanding of the ecological interactions.This example demonstrates that humans are1 JMBA Global Marine Environment

keystone members of the seagrass communityin Puerto Aldea. Enormous shells heaps frompre-Columbian times provide testimony that thearea has been a rich fishing ground forcenturies before the arrival of the colonists(Figure. 3). The persistence of the seagrassbeds and its associated fauna until nowsuggests that human activities have not yetinterfered with this unique ecosystem. Newtechnological advances could change thisscenario, however.FUTURE FOR THE UNIQUE SEAGRASSMEADOW OF PUERTO ALDEAUrban development and aquacultureactivities are intensified in the vicinity of PuertoAldea. Improved infrastructure and the arrivalof running water might bring increasingnumbers of tourists. These increasing externalpressures might affect the integrity of theseagrass meadow in the future. The localfishermen are aware of the importance of theseagrass meadow and they have taken care ofit well in the past. They have repeatedlyexpressed both in word and action theirinterest in maintaining this unique place. It is tohope that they will be able to increase theirown living standard (e.g. by having runningwater in their homes) while at the same timeconserving their natural scallop garden.Figure 1. The seagrass meadow and Puerto Aldea 50 Km south ofCoquimbo, Chile.FIGURE 3. Shell heaps from pre-Colombian times in the vicinity of Puerto Aldea.Recent articles in JMBA:Sánchez-Jerez, P., Barberá-Cebrián, C. & Ramos-Esplá, A. Dailyvertical migrations in the epifauna associated with Posidoniaoceanica meadows. JMBA 79, 971-977Guidetti, P. Invertebrate borers in the Mediterranean sea grassPosidonia oceanica: biological impact and ecological implications.JMBA 80, 725-730Burns, D. &. Bingham, B. L. Epibiotic sponges on the scallopsChlamys hastata and Chlamys rubida: increased survival in a highsedimentenvironment. JMBA 82, 961-966Navarro, J. M., Fernández-Reiriz M. J. & Labarta, U. Short-termfeeding response of the scallop Argopecten purpuratus exposed totwo different diets. JMBA 84, 775-779JMBA Global Marine Environment 2