Female flowers of Enhalus acoroides
The periodic forays by teamseagrass to Singapore's coastal habitats this year are part of a global research project to monitor the growth and health of seagrass meadows. Though they appear unremarkable at first sight, seagrasses are botanical retrogrades: flowering plants from about six Monocotyledon families that have made their way back to the sea after an extended phylogeny of land-locked, and later, freshwater-living lineages. Unlike seaweed (which belong to a few entirely separate kingdoms altogether – only green seaweed share the plant kingdom with mosses, ferns and flowering plants) which reproduce via spores, seagrasses bear flowers and fruits in addition to vegetative propagation through their rhizomes and stolons (creeping stems that spread out over or through the substrate and take root).
Seagrasses at Semakau
Though they seem structurally unremarkable and are barely known even to many ecologists, much less the public, seagrass habitats are reckoned to be among the richest and most productive ecosystems on the planet. The plants themselves provide homes for myriad creatures from sea anemones to seahorses and serve as nurseries for young fish, shrimp and crabs before they get large enough for the open sea. Dugongs and turtles feed on the seagrasses and in turn fertilise the field with their droppings. These aquatic savannahs also sequester carbon and act as sponges that soak up nutrients and filter the water, while their spreading mesh of roots and rhizomes bind the sediment and help maintain the stability of the sea floor. For adjacent coral reefs that rely on the sun's rays for sustenance, this service can be vital in preserving the quality and clarity of the water. The health of seagrass meadows hence provides a good proxy of water and other environmental conditions (click to download a pdf article), as declining meadows may indicate that conditions are deteriorating too rapidly for the plants to cope with.
Besides their ecosystem services, some seagrasses are edible. The crunchy seeds of Enhalus acoroides reek of water chestnuts. Their fibrous foliage is also used in insulation, roofing, packaging and craftswork (see this page on the human uses of seagrass).
The mud flats off Chek Jawa and Pulau Semakau (above) boast probably the largest expanse of seagrasses in Singapore, with Cyrene Reef (pictured below and now doomed to be buried under a petrochemical storage facility) coming close in biological richness. The sandy shores of Changi also harbour ample growths of spoon seagrass (Halophila ovalis) and fern seagrass (Halophila spinulosa), while lesser colonies can be found off Siloso on Sentosa, Tuas, Labrador Park and the Southern Islands. About 11 species of seagrasses (out of 60 known species globally) are known locally, including the rather rare Halophila beccarii. Teamseagrass's periodic sessions at these localities provide comparable data on the range and spread (or decline) of local seagrass species as well as trends in algal and epiphyte growth (which indicate the presence of excess pollutantsnutrients) to help evaluate the health of the meadows as well as the surrounding littoral habitats.
Singapore probably can't afford to care much about her natural maritime heritage (there being no single Marine Protected Area in the entire country). Her economy relies almost entirely on the ingenuity of human capital whose projected population growth is easily sustained by turning every available undeveloped plot of land and sea into housing and commercial facilities. Everything else can be imported, including whale sharks.
Lower nations such as Australia are less fortunate. Heavily reliant on primary production in the fruit of the land and spawn of the sea, such economies also wrestle with inconvenient struggles between the need to preserve pristine coastlines which draw visitors and improve the quality of life for maritime communities and the lure of developing them into exclusive resorts and civilised seawalls that keep fragile homesteaders safely away from the untameable waves.
The shallow waters off Adelaide in Southern Australia cover vast expanses of temperate seagrasses that contribute the region's near-shore fisheries productivity, seabed stability and biodiversity. However, since the 1940s, over 5,000 ha of near-shore seagrass meadows have been lost. Sasi Nayar of the South Australian Research and Development Institute (SARDI) was in Singapore recently to present a summary of the work his team has done over four years to uncover the cause(s) of this decline. Previously, Sasi was based in Singapore where he researched pollutant dispersal in Ponggol River (a rather nasty piece of work it seems, judging by the conclusions – the river not the thesis that is) for his doctorate and now he works in Adelaide, where people actually pour resources into efforts to find out why ecosystems are deteriorating and even try to fix the causes.
Up to 13 species of seagrasses are known to occur along Adelaide's metropolitan coastline, which stretches for over 80 nautical miles and cover the inter-tidal zones down to depths of 30-40 m. Ecologically, the seagrasses there exhibit fairly broad tolerance limits, with some able to withstand floods of freshwater (which tends to screw up the osmotic balance of obligate marine organisms). The meadows harbour highly diverse associate communities of other creatures, including 13 species of commercially-important fishes, making them a productive nurseries for the area's fisheries.
Two near-shore subtidal seagrass species with varying physiological characteristics were selected as study subjects to help investigate this ecological disaster: Amphibolis antarctica (pictures c and d) and Posidonia australis (pictures a and b). P. australis (45 cm high) has a relatively high light requirement, strap-like leaves and extensive rhizome networks that spread out basally. Amphibolis (1 m high) is woodier and exhibits apical growth, with stems that terminate in small frond-like leaves and relatively small rhizome systems. It also absorbs nutrients primarily through the leaf blades than the roots, rendering it particularly susceptible to high nutrient loading and toxicity (i.e. when quantities of nutrients such as ammonium and phosphate exceed the plant's ability to bind or metabolise them and reach damaging levels). Species from these two genera have also suffered the greatest losses.
In the 1970s, greater access to scuba gear offered divers and ecologists a chance to directly observe seagrass declines, as they saw meadows becoming increasingly fragmented and more sites devastated by 'blow-outs': gouges of substrate that migrate around the seabed to smother or rip off seagrasses faster than the plants' natural rate of recolonisation. The main losses occurred in a 1.2 km wide strip parallel to Holdfast Bay and two other sites: around the Port Adelaide sludge outfall and a dredge spoil-dumping ground off Adelaide's Outer Harbour.
Sasi revealed that when the seagrass is lost, their role as nutrient sponges is rapidly taken over by phytoplankton and algae which soak up the excess nutrients in the water. Animals that rely on the seagrass for food and shelter disappear and the much simpler algae-dominated habitats experience severe falls in biodiversity. Worse, the advent of the algae means the remaining seagrasses face a self-perpetuating cycle of decline in a negative feedback loop where substrate erosion caused by seagrass deaths inundates nearby colonies and leads to further losses.
Freed from the seagrass roots, the seabed becomes less stable. Sand movement increases, especially during storms, and threaten to choke marinas. More dredging is consequently needed to clear the seabed to allow for the passage of vessels.
The question facing Sasi's team was puzzling. In places like Florida, seagrass losses showed a progression that started offshore and moved into near-shore zones, with propeller damage and dredging/filling work being the key causes. The opposite model applied to Adelaide, possibly indicating a terrestrial culprit. Hypothesised villains included effluent from wastewater treatment plants and factories, stormwater runoff from agricultural areas, excessive freshwater flows, sedimentation from land-based activities and toxicants. The possibility of an interplay between multiple factors was not ruled out.
Sasi wondered if toxicants such as herbicides, pesticides and heavy metals from the upper reaches of the Murray River could have played a role in the seagrass deaths in blowout zones. Over at Queensland, runoff from sugar and banana plantations has been implicated in damage to the Great Barrier Reef. However, sediment and water samples collected over a year off Adelaide found little evidence to support this idea. One of the studies (out of some 20+ reports that spanned Sasi's study) noted though that petrochemicals (hazardous materials and effluent from refineries, leakage and spills from oil facilities) can have significant impact on seagrasses in the intertidal zone in direct smothering or phototoxicity that reduces the plants' tolerance to other stress factors.
Halophila beccarri at Chek Jawa
Salinity was examined next, as freshwater that reaches the sea is typically turbid and filled with land-based pollutants and nutrients. In Singapore, Halophila beccarii has been observed to thrive in zones that received heavy freshwater runoff such as the mouths of rivers following floods and mangrove swamps, and periods of lower salinity have been suggested to be necessary triggers for the species' flowering. Other seagrasses such as Cymodocea spp. and Halodule spp. appear to fare badly after seasons of intensive rain and flooding, however.
In situ and laboratory tests over both long and short periods using PAM flourometers measured photosynthetic activity in seagrasses at different stages of their lifecycle. At depths of about 50m, the researchers found no significant fluctuations in salinity. In shallower zones, there were seasonal shifts between summer and winter, but Sasi noted that while he could demonstrate an influence on growth from reduced salinity at very low salinity levels over sustained periods, this influence does not take place in the field "except at an extremely localised scale" of 10s of square metres. Salinity was therefore ruled out.
Dredging to deepen seabeds or reclaim land (as well as sediment plumes from rivers after storms) causes huge increases in turbidity. The reduced light and high levels of suspended sediments spells doom for many organisms that need to photosynthesise or filter-feed. Particles that settle on leaves block further access to light, hindering or stopping growth altogether. The SARDI team installed light meters at depths at contours of 5m, 10m and 20m along the coast for 15 days. They found on average no light limitations at the near-shore zones of 3-5m, where seagrasses have declined the most and so discounted light as a contributing factor.
The team then looked at the possibility of eutrophication or nutrient excesses. This refers to a surplus of nutrients (particular nitrogenous and phosphoric compounds) that reach levels toxic to the plants themselves and that also allow algae to become established and smother the seagrasses. Epiphyte growth on seagrass leaves (which teamseagrass measures as well in Singapore) becomes an indicator of water conditions that become more favourable to algae than seagrass. The established model of "nutrient spike >> epiphyte boom >> seagrass decline" can take place in just a month
According to Sasi, Southern Australian waters are naturally oligotrophic or nutrient-poor. Excess nutrients therefore originate from land-based inputs such as chemical plants, agriculture and wastewater treatment facilities.
In situ validation of the effects of eutrophication were performed by seeding nutrients at selected pristine meadows of Amphibolis and Posidonia. Three plots (20m apart) were fertilised and three served as controls.
After about two months, the seeded Posidonia were choked by algal growth (see picture left) and epiphytes that blocked light and gas exchanges on the leave blades. After six months, the seeded plots of Amphibolis (which had their apical growing tips swamped by epiphytes) were reduced to bare sand. For comparison, controlled mesocosm trials were also performed in the lab to simulate the effect of differing light and nutrient levels on the seagrasses. At the same, monitoring studies showed that the near-shore zones off Adelaide experience very high nutrient loading levels.
Sasi revealed that hydrodynamic modelling exercises demonstrate negligible mixing of near-shore waters with offshore zones. Instead, the shallow water mass (down to 6m) tends to move up and down perpendicularly to the shoreline.
Eutrophication in the water column leading to heavy epiphyte loads thus appears to be the key factor for the seagrass decline. Other associated factors were thought insignificant, as shading from phytoplankton blooms would be highly localised and rapidly dispersed by water currents, while no toxic response to elevated nutrients was observed in the seeded plots, which would have been evidenced by reduced leaf growth. Grazing activity (snails, sea stars, sea urchins) was also reckoned to be of minimal impact and more focused on the epiphytes rather than the tough seagrass foliage.
Thus, Sasi concludes that smothering by epiphytes, especially on vital budding regions of the seagrasses, has caused increased necrosis and sloughing of seagrass material at rates that exceed the rate of new growth and ultimately exhaust the reserves stored in the stems and roots. The average of length of leaves in seeded plots also decreased, suggesting an increasing rate of epiphyte-mediated sloughing that prevents the leaves from reaching their full length. The net cumulative effect is death for the affected region of seagrass and the fieldwork indicated that while the seeded plot may share a rhizomic network with adjacent healthy plots, photosynthates (products of photosynthesis) were not being transferred (or at insufficient rates) to the degraded areas.
Natural seasonal cycles that dump a surge of nutrients into seagrass beds also lead to periodic blooms of epiphytes, but the high nutrient load is short-lived and the seagrasses survive by tapping their belowground reserves. The Adelaide die-offs were hence related to prolonged anthropogenic activities by the coast or further inland.
What next? The team now had to model the total biological and resource allocation of anthropogenic nutrients to develop a loading model that shows the biologically assimilable limits for nutrients in seagrass communities.
This was done using in situ clear incubation chambers that measured the biological uptake of various nutrients by seagrasses. Unlike our leisurely intertidal strolls over muddy flats and
mosquito-infested swamps, Sasi's dive team had to ferry crates of
fragile equipment through cold and cloudy water while dodging vicious swimming crabs and the
occasional great white shark, a local fish known to have a taste for
marine biologists.
The team enriched the chambers with varying levels of nutrients and measured the plants' response to these spikes over time. The results were scaled up to match the known nutrient levels and biotic mass of the Adelaide coast to attain comparative estimates of the total uptake rates for the different nutrients.
For ammonium (NH4), seagrasses and their epiphytes were estimated to be taking up about 31% of the total anthropogenic input into the metropolitan coastline (a total of 1,509 tons of ammonium are dumped each year). The surplus is taken up by free-living algae and plankton. All in all, the seagrass complex (comprising seagrass leaves, stems, roots and epiphytes) accounted for 98% of the total biological assimilation of ammonium. About 474 tons of nitrate (NO3) are dumped in Adelaide's metropolitan coastline each year. Seagrasses were found to account for 88% of the coast's assimilative capacity for nitrate, but less than 1% of the total nitrate dumped is absorbed.
Thus, seagrass systems play a major role in the nitrogen cycle of Adelaide's coastline, acting as sinks to bind, metabolise and recycle nutrients that would otherwise be consumed by algae. However, the total volume of nutrients (nitrate, ammonium and phosphate) dumped into the sea is far beyond their absorptive capacity. This suggest that the considerable nutrient excess in the water column may have as yet unclear impact on the stability of the broader near-shore ecosystem, a condition that could be exarcebated by the seagrass declines caused to the eutrophication itself – a double whammy as it were.
Following the reports by Sasi's team, steps were taken by the state environmental agency to mitigate effluent outflows from industrial plants in the region. But recovery for Adelaide's seagrasses, given the decades of inorganic waste that swirls in their midst, is likely to be slow and the subject of future studies.
Links to papers by Sasi Nayar's team at SARDI: http://www.sardi.sa.gov.au/pages/aquatic/pub/mee/mee_pub.htm:sectID=1082&tempID=1
Good post! Keep 'em comming!
Posted by: romunov | 13 August 2007 at 02:12 AM