Short Term Sinks ie Material Broken Down Over a Period of a Year or so

The preliminary compilation made for this chapter showed that the percentage of seagrass production accumulated as refractory material varies from ca. 1 to 62% and the absolute value from 3 to 207 gC m-2 year-1 (Fig. 3G and H). The studies on P. oceanica presented in this chapter (for beds off the islands of Medes, Calvi, and Ischia; Table 3) and estimates from two other available studies (Cebrian et al., 1997; Gacia et al., 2002), indicate that the average potential carbon sink (short-term) from leaf-derived material for this species can be estimated as

Table 4. Estimates of the potential annual carbon sink in Posidonia oceanica meadows and other environments in absolute values and relative to net primary production (% NPP; adapted and simplified from Mateo, submitted). Estimates are presented separately for leaves, below-ground organs, total plant and total ecosystem (see text for details).

Potential sink

Table 4. Estimates of the potential annual carbon sink in Posidonia oceanica meadows and other environments in absolute values and relative to net primary production (% NPP; adapted and simplified from Mateo, submitted). Estimates are presented separately for leaves, below-ground organs, total plant and total ecosystem (see text for details).

Potential sink

Posidonia oceanica

Aboveground

Medes

5

123

59

Medes

13

22

23

Calvi

20

17

11

Cala jonquet

4

66

17

Fanals point

15

40-55

36-50

Ischia

5-20

20

20

Average

12

57

29

Range

4-20

17-123

17—59

Belowground

Short-term

Medes

5

84

91

Medes

13

24

96

Long-term

5-20

9-112

4-51

Total plant

Medes

5

207

63

Medes

13

46

42

Ischia

5-20

58

43

Total ecosystem

Fanals point

15

182

43

Calvi

20

56

45

Others

Peatlands

-

26-99

14-52

Coastal areas

-

62

50

Oceanic areas

-

0.02-4

0.01-2

Seagrass ecosystems

0-40

3-182

1-62

57 gC m-2 year-1 (ranging 17-123 gC m-2 year-1) representing 29% of the annual leaf production (1759%, Table 4). This estimate accounts for a substantial part of the range estimated for all seagrasses, suggesting that the phenomenon of refractory accumulation may present large between- and within-species variability.

As discussed above, carbon from below-ground organs may also contribute to the sediment refractory pool. The carbon stored in a P. oceanica bed after the first year of decay (short-term sink) was estimated to be 84 and 24 gC m-2 year-1 at the shallow and deep limits of the bed, respectively (Mateo and Romero, 1997); this represents 91 and 96% of the total below-ground production, respectively (Table 4). Adding up both above- and below-ground sinks for the P. oceanica bed in Medes Islands, the total potential sink for the studied year amounted to 207 and

46 gC m-2 year-1 at 5 and 13 m, respectively (63 and 42% of total plant production, Table 4). Comparable estimates were only possible for the bed in Ischia Island that yielded an average of 58 gC m-2 year-1 (43% of total plant production; average for 5-20m depth).

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