Ecosystem Services

of 29% of the seagrass area represents therefore a major loss of natural carbon sinks in
the biosphere (Duarte et al. 2010).
By using the best available estimates of carbon burial rates in seagrass meadows,
Kennedy et al. (2010) calculated that between 41 and 66 g C per m 2 per year originates
from seagrass production. In addition, they estimated that total carbon burial in
seagrass meadows was 48–112 Tg per year when including global average for allochthonous
carbon trapped in seagrass meadows. This shows that seagrass meadows are
natural hot spots for carbon sequestration.
4.5.3 Eutrophication mitigation
Due to nutrient cycling and storage, eelgrass by enhancing denitrification, minimizes
the efflux of ammonia and phosphate effluxes to the water column, cleans the water
and mitigates eutrophication, and possibly reduces growth of opportunistic macroalgae
and phytoplankton.
In a Swedish valuation scenario, Cole and Moksnes (2016) calculated that the value
of nitrogen storage derived from a hectare of eelgrass to be approximately SEK 5,600
(USD 680) annually, based on a nominal removal of 466 kg of nitrogen by the eelgrass
and the cost to the society of removing an equivalent amount.
4.5.4 Water purification, filtering and removing of hazardous substances
Eelgrass absorbs nutrients from the water column for their growth and reproduction
(see Chapter 4.3.2 for references). Uptake of nutrients by eelgrass and other submerged
aquatic vegetation (SAVs) can help to prevent nuisance algae blooms and can improve
water clarity. The presence of eelgrass therefore helps mitigate the impact of excessive
nutrient input to the estuary from human activities.
Eelgrass may play an important role in biogeochemical cycling of heavy metals and
several works have studied uptake and translocation of heavy metals, among Lyngby
and Brix (1982, 1989) and Ferrat et al. (2012).
The plant tissue of eelgrass significantly accumulates high levels of heavy metals
when growing on heavy metal-impacted sites. In Puget Sound (USA) eelgrass
above/below ground biomass is estimated to 10/5 million kg, respectively. Total accumulation
of metals was estimated to be 300/30 kg copper, 2/280 kg lead, and
800/0.4 kg zinc respectively in above/below ground biomass. Three to 10 times calculated
aboveground value may be cycled or stored in one year due to growth and shed
of old leaves during the growth season.
62 Ecosystem Services