France - The Mediterranean Lagoons
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with the land run-off. Particularly affected are shallow lagoons.· Internal
sources are mainly intensive mariculture systems such as fish and shellfish farms. While agricultural waste waters contain mainly nitrate and
domestic wastes are dominated by phoshorus, intensive mariculture contributes to the ammonia input into the lagoons.
Seasonal changes in the nutrient cycle of a lagoon can be summarized
as follows: (1) nitrate levels are mainly controlled by the fresh-water inputs. They show therefore a maximum at the end of periods of heavy
rainfall, i.e. at the end of spring in the Mediterranean lagoons (De
Casabianca-Chassany et al. 1990). (2) Likewise phosphate levels are greatly
controlled by fresh-water inputs and by deposition in the sediments. (3)
Ammonium-nitrogen originates from biodeposits on the sediment surface.
(4) Both phosphate and ammonia are released at the end of summer when
anoxic conditions occur. (5) Due to these seasonal variations, the N:P ratio
in the water may change considerably from 16 to 1.
13.3.1 Languedocian Lagoons
Compared to the Corsican lagoons that do not suffer from urban pollution and for which the agricultural exploitation of the surrounding areas
has only just begun, the Languedocian lagoons (Fig. 13.8; Table 13.1) are
characterized by the influence of domestic discharge (Fig. 13.9). These
carry high loads of phosphate which is stored in the sediment and may
be released during periods of distrophia causing very high concentrations (> 1 mg dm -3 phosphate; De Casabianca-Chassany 1977).
13.3.1.1 Thau Lagoon
In the Languedoc region, the Thau lagoon is a large tectonic, marine
lagoon of 7500 ha, stretching over 15 km parallel to the sea (Fig. 13.10).
The average depth is 5 m (maximum depth 10m). The lagoon which is
connected with the Mediterranean Sea by the Sete and Marseillan canals
is the site of intensive oyster farming and is subject to periods of severe
distrophia during the summer. There are various zones of eutrophication
induced by occasional nutrient inputs due to human activity: (I) shellfish farming zones covering 1500 ha. Faecal biodeposition from mussels
and oysters causes a predominance of NH 4 -N. (2) Fresh-water input
causes increased nitrate and nitrite levels in the eastern part of the
lagoon, whereas increased phosphate concentrations are due to wastewater discharge in the north at Meze. (3) Particulate matter input
(construction of the Moure-Blanc port, and industrial zone to the SE).
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with the land run-off. Particularly affected are shallow lagoons.· Internal
sources are mainly intensive mariculture systems such as fish and shellfish farms. While agricultural waste waters contain mainly nitrate and
domestic wastes are dominated by phoshorus, intensive mariculture contributes to the ammonia input into the lagoons.
Seasonal changes in the nutrient cycle of a lagoon can be summarized
as follows: (1) nitrate levels are mainly controlled by the fresh-water inputs. They show therefore a maximum at the end of periods of heavy
rainfall, i.e. at the end of spring in the Mediterranean lagoons (De
Casabianca-Chassany et al. 1990). (2) Likewise phosphate levels are greatly
controlled by fresh-water inputs and by deposition in the sediments. (3)
Ammonium-nitrogen originates from biodeposits on the sediment surface.
(4) Both phosphate and ammonia are released at the end of summer when
anoxic conditions occur. (5) Due to these seasonal variations, the N:P ratio
in the water may change considerably from 16 to 1.
13.3.1 Languedocian Lagoons
Compared to the Corsican lagoons that do not suffer from urban pollution and for which the agricultural exploitation of the surrounding areas
has only just begun, the Languedocian lagoons (Fig. 13.8; Table 13.1) are
characterized by the influence of domestic discharge (Fig. 13.9). These
carry high loads of phosphate which is stored in the sediment and may
be released during periods of distrophia causing very high concentrations (> 1 mg dm -3 phosphate; De Casabianca-Chassany 1977).
13.3.1.1 Thau Lagoon
In the Languedoc region, the Thau lagoon is a large tectonic, marine
lagoon of 7500 ha, stretching over 15 km parallel to the sea (Fig. 13.10).
The average depth is 5 m (maximum depth 10m). The lagoon which is
connected with the Mediterranean Sea by the Sete and Marseillan canals
is the site of intensive oyster farming and is subject to periods of severe
distrophia during the summer. There are various zones of eutrophication
induced by occasional nutrient inputs due to human activity: (I) shellfish farming zones covering 1500 ha. Faecal biodeposition from mussels
and oysters causes a predominance of NH 4 -N. (2) Fresh-water input
causes increased nitrate and nitrite levels in the eastern part of the
lagoon, whereas increased phosphate concentrations are due to wastewater discharge in the north at Meze. (3) Particulate matter input
(construction of the Moure-Blanc port, and industrial zone to the SE).