Biogenic sediments from coastal ecosystems to beach–dune
systems: implications for the adaptation of mixed and carbonate beaches to future sea level rise

Coastal ecosystems produce and store carbonate
particles, which play a significant role in the carbonate dynamics
of coastal areas and may contribute to the sediment
budget of adjacent beaches. In the nearshore seabed of temperate
zones (e.g. Mediterranean Sea and South Australia),
marine biogenic carbonates are mainly produced inside seagrass
meadows. This study quantifies the contribution of
biogenic sediments, mainly produced in Posidonia oceanica
seagrass meadows and secondarily in photophilic algal communities,
to the sediment budget of a Mediterranean beach–
dune system (San Giovanni beach, western Sardinia, western
Mediterranean Sea). A set of geophysical, petrographic and
sedimentological data was used to estimate the sediment volume
and composition of the beach–dune system as a whole.
The San Giovanni beach–dune system contains 3 797 000 ±
404 000 t of sediment, 83 % (3 137 000 ± 404 000 t) of which
is located in the coastal wedge, 16 % (619 000 ± 88 000 t)
in the dune fields and 1 % (41 000 ± 15 000 t) in the subaerial
beach. The sediments are composed of mixed modern
bioclastic and relict bioclastic and non-bioclastic grains
from various sources. The system receives a large input of
modern bioclastic grains, mainly composed of rhodophytes,
molluscs and bryozoans, which derive from sediment production
of present-day carbonate factories, particularly P.
oceanica seagrass meadows. Radiocarbon dating of modern
bioclastic grains indicated that they were produced during
the last 4.37 kyr. This value was used to estimate the longterm
deposition rates of modern bioclastic sediments in the
various beach compartments. The total deposition rate of
modern bioclastic grains is 46 000±5000 t century−1
, mainly
deposited in the coastal wedge (39 000 ± 4 000 t century−1
)
and dunes (7000±1000 t century−1
), and 46 000 t represents
∼ 1.2 % of the total beach–dune sediment mass. Carbonate
production from coastal ecosystems was estimated to be
132 000/307 000 t century−1
, 28 % (15 % / 34 %) of which is
transported to the beach–dune system, thus significantly contributing
to the beach sediment budget.
The contribution to the beach sediment budget represents
a further ecosystem service, which our data can help quantify,
provided by P. oceanica. The value of this sedimentsupply
service is in addition to the other important ecological
services provided by seagrass meadows. The dependence
of the beach sediment budget on carbonate production associated
with coastal ecosystems has several implications for
the adaptation of mixed and carbonate beaches to the loss of
seagrass meadows due to local impacts and the changes expected
to occur over the next few decades in coastal ecosystems
following sea level rise.