Seagrass Systems Under Nutrient Loads and Grazing

FACTs

Most people I tell about my research subject didn’t knew seagrass existed! So why is seagrass worth all this government research money? And why are Green turtle’s involved? Here some facts:

Seagrasses stabilizing shorelines and act as a nursery for a lot of (coral reef) species. And it’s just beatifull!
Seagrass isn’t seaweed! Seagrasses produce flowers, fruit and seeds, algae produce spores. Seagrasses, have separate roots, leaves and underground stems called rhizomes. Unlike algae it can survive in nutrientpoor waters by getting nutrients below the surface.
There are 49 species of seagrasses (Den Hartog 1970). Present from temporal to tropical waters.
Over a billion people live within 50 m of a seagrass meadow. Millions of people obtain their protein from animals that live in seagrass.
The estimated covarage of seagrasses globally is over 177000 square kilometers
A hectare of seagrass absorbs 1.2 kg of nutrients per year, equivalent to the treated effluent from 200 people (McKenzie 2008)
1 square metre of seagrass can produce up to 10 liters of oxygen per day.
Seagrass occupy only 0,1% of the seafloor, yet are responsible for 12% of the organic carbon burried in the ocean, which helps reduce greenhouse gases. (Educators handbook Seagrass Watch 2009)
Human activities near shorelines (eutrophication, mining, forest clearing) result in a rapid decline of seagrasses.
Seagrass is the primary diet of green turtles (Bjorndal 1997) and many more.
At my research site (Derawan archipelago), Green turtles are still present in high densities (I counted 15 per hectare), grazing seagrass and nesting at beaches of Derawan, Sangalaki and other islands.

Did you know that, Green turtles (Chelonia mydas; Linnaeus 1766):

Named “Penyu” (for all seaturtles) in Bahasa Indonesia, “Soepschildpad” in Dutch.
It’s green name is not for the color of its shell, which is normally brown or olive depending on its habitat, but for the greenish color of its skin.
Derawan Island is surrounded by seagrass meadows meadows dominated by Green turtles favourite food: Halodule uninervis and Halophila ovalis
1 – 5 Turtles come to Derawan Island each evening (year round) to lay their eggs!
WWF Turtle guards dig out the eggs and put in in a hatchery (beach surrounded by wooden fence).
Almost every night you can see a nest hatch and help the guards to release the little Tukik (Bahasa for baby turtle) to the sea.
Turtle guards are necessary since there is still a big trade in turtle eggs, you can buy them on Samarinda’s streets.
Don’t buy Turtle souvenirs: Many of the bracelets and rings sold on Derawan are from the shell of a (killed!) hawksbill turtle.
The shell is cut and the turtle is released again, but off course the shell will not grow back and the turtle will die!
Survided since the Cretaceous period, 145 million years ago. In comparison: human genetic studies now suggest a divergence of the Neanderthaler species from Homo sapiens about 500,000 years ago (Green 2006, Nature)
This herbivorous reptile has an average lifespan in the wild of over 80 years
Females reach maturity at an average age of 40 years (range 20-50y) (Limpus & Chaloupka 1997)
Maximum size up to 1,4 meters and 310 kilograms.
Migration from foraging to their natal nesting beach ranges over 1000km (Carr & Ogren 1960, Meylan 1982, Limpus et al. 1992)
They will come back to their breeding ground every 2 – 6 years (Carr & Carr 1972, Solow et al. 2002, Limpus et al. 2003)
If they breed they can have 3–9 clutches (Johnson & Ehrhart 1996, Bjorndal et al.1999, Limpus et al. 2003), coming back to lay their eggs every 15 days. Only 25% of the turtles will have more than 1 clutch.
Average clutch size can be up to 106 eggs (Aru, Dethmers 2009) & positively correlates with body size.
Incubation time depends on temperature is on average 59 days (±5 Dethmers 2009), 85% successfully hatches.
Temperature also affects the sex of the progeny (Mrosovsky 1997).
Young turtles hatch usually at night and they start moving towards the brightest horizon, usually to the open sea.
Where they go and what they do during their juvenile years is very poorly known.
Status: threatened with extinction (CITES Appendix. I)
Food sources: young greens are onmivores. Later they feed on algae and seagrass, but also on jellyfish, salps (tunicates) and sponges.
Distribution: Is found circumglobally on tropical and subtropical parts of the Atlantic, Pacific and Indian Ocean.
Prefers to stay close to shallow, coastal areas and reefs where seagrass and algae grow.
In Indonesia only four nesting areas are known to host more than several hundred nesting C. mydas every year; Aceh (north Sumatra), Pangumbahan (west Java), Berau Islands (east Kalimantan), and the Aru Islands (southwest Moluccas).
Intensive egg harvests occurred at virtually all nesting beaches throughout the Indonesian Archipelago (Sloan et al. 1994, Wicaksono 1992, Polunin 1975, Limpus 1997).
Now there is a egg-harvest concession at Pangumbahan, southwest Java (Sloan et al. 1994) and Berau, east Kalimantan (Wicaksono 1992)
Current sea turtle abundance is believed to be only a fraction of its historical (14th century) population sizes (Bjorndal & Jackson 2003).
Worldwide, Green turtle numbers decline because a i) loss of critical nesting habitat as a result of human encroachment, ii) loss of feeding habitat through destructive fishing practices, iii) by-catch in trawl fisheries and iv) entanglement in discarded drift nets.
Drier and warmer conditions of an El Niño event reduce the productivity at foraging areas, due to higher risk off collapse egg-pits due to drier sand.
Removing these large herbivores from marine ecosystems has far reaching effects (as summarized in Bjorndal and Jackson 2003).
Major changes in a grazing regime can be expected to result in major changes in biodiversity, productivity and structure of seagreass beds: For example,
- ungrazed seagrass beds shorten nutrient cycling times reducing primary productivity (Thayer et al. 1984);
- increase particle entrapment and substracte depositions, altering the physical structure of important fish-nurseries (Jackson et al. 2001);
- and increase the chance of sulfide toxicity and infection by epibionts (Harvell 1999).
A lot of turtle facts were summarized in the thesis of Dethmers.