51
beaches and dunes takes place on the right side due to the prevailing northwest
wind direction (Cearreta et al. 2004). The Santoña estuary is formed by the tidal
part of the Asón River. The 11-km long and 0.5 to 3-km wide estuary covers an
area of 2000 ha (Cearreta 1988). Around 63% of the total surface is exposed at
low tide (Irabien et al. 2008b). The eastern part of the lower estuary is dominated
by sand, where beaches and sand dunes develop, whereas the western part is
mainly made of mud, where salt marshes evolve. The Plentzia estuary is formed
by the tidal part of the Butroe River. The estuary covers a total area of 115 ha, with
a 7 km length and an average width of 20 m (Gobierno Vasco 1998). Around 80%
of the estuarine surface is exposed at low tide (Cearreta et al. 2002). Nowadays,
salt marshes develop in a few isolated patches in the lower and upper estuary.
Finally, the easternmost Urdaibai estuary has a total surface of 765 ha, where tidal
waters are mixed with fresh waters from the Oka River. The estuary covers a
12-km long and 1-km wide alluvial valley (Monge-Ganuzas et al. 2011). The
lower part of the estuary is dominated by sand, while in the middle and upper
parts mud becomes dominant and salt marshes are more abundant. The three estuaries present a semidiurnal tidal regime and a mean tidal range of 2.5 m (Cearreta
et al. 2008) that fluctuates between a minimum oscillation of 1 m during neap
tides to a maximum variation of 4.5 m during spring tides (Villate et al. 1990;
Monge-Ganuzas et al. 2008).
2.3 Collecting and Sampling Sedimentary Records
Sediment cores were collected from the central part of the salt marsh, far from
wave action. Two 50-cm long and 12.5-cm diameter PVC tubes were inserted by
hand into the sediment at each sampling point and the extracted core length
depended on soil resistance (Fig. 2.2). Compaction of the sediment during sampling was negligible due to the minerogenic nature of the sediment in northern
Spain. In fact, there is an important regional input of minerogenic material carried by estuarine waters that exceeds by far the organic matter derived from
salt-marsh vegetation, which is rapidly consumed with depth (Cearreta
et al. 2002, 2013; Fernández et al. 2010). Precise location and elevation of the
cores was measured in the field using a Global Positioning System-Real Time
Kinematic (GPS-RTK) or a total station, and referenced to the local ordnance
datum (LOD: lowest tide at the Bilbao Harbour on September 27, 1878), which
is located 1.727 m below the national leveling datum (Mean Sea Level in
Alicante or NMMA).
In the laboratory, we divided each core replicate in halves and obtained four
identical core replicates to carry out different analyses (i.e. foraminifera, sand,
short-lived radionuclides, and heavy metals). Each replicate half was then subdivided in 1-cm-thick samples for analysis.
2 Recent Agricultural Occupation and Environmental Regeneration of Salt...
beaches and dunes takes place on the right side due to the prevailing northwest
wind direction (Cearreta et al. 2004). The Santoña estuary is formed by the tidal
part of the Asón River. The 11-km long and 0.5 to 3-km wide estuary covers an
area of 2000 ha (Cearreta 1988). Around 63% of the total surface is exposed at
low tide (Irabien et al. 2008b). The eastern part of the lower estuary is dominated
by sand, where beaches and sand dunes develop, whereas the western part is
mainly made of mud, where salt marshes evolve. The Plentzia estuary is formed
by the tidal part of the Butroe River. The estuary covers a total area of 115 ha, with
a 7 km length and an average width of 20 m (Gobierno Vasco 1998). Around 80%
of the estuarine surface is exposed at low tide (Cearreta et al. 2002). Nowadays,
salt marshes develop in a few isolated patches in the lower and upper estuary.
Finally, the easternmost Urdaibai estuary has a total surface of 765 ha, where tidal
waters are mixed with fresh waters from the Oka River. The estuary covers a
12-km long and 1-km wide alluvial valley (Monge-Ganuzas et al. 2011). The
lower part of the estuary is dominated by sand, while in the middle and upper
parts mud becomes dominant and salt marshes are more abundant. The three estuaries present a semidiurnal tidal regime and a mean tidal range of 2.5 m (Cearreta
et al. 2008) that fluctuates between a minimum oscillation of 1 m during neap
tides to a maximum variation of 4.5 m during spring tides (Villate et al. 1990;
Monge-Ganuzas et al. 2008).
2.3 Collecting and Sampling Sedimentary Records
Sediment cores were collected from the central part of the salt marsh, far from
wave action. Two 50-cm long and 12.5-cm diameter PVC tubes were inserted by
hand into the sediment at each sampling point and the extracted core length
depended on soil resistance (Fig. 2.2). Compaction of the sediment during sampling was negligible due to the minerogenic nature of the sediment in northern
Spain. In fact, there is an important regional input of minerogenic material carried by estuarine waters that exceeds by far the organic matter derived from
salt-marsh vegetation, which is rapidly consumed with depth (Cearreta
et al. 2002, 2013; Fernández et al. 2010). Precise location and elevation of the
cores was measured in the field using a Global Positioning System-Real Time
Kinematic (GPS-RTK) or a total station, and referenced to the local ordnance
datum (LOD: lowest tide at the Bilbao Harbour on September 27, 1878), which
is located 1.727 m below the national leveling datum (Mean Sea Level in
Alicante or NMMA).
In the laboratory, we divided each core replicate in halves and obtained four
identical core replicates to carry out different analyses (i.e. foraminifera, sand,
short-lived radionuclides, and heavy metals). Each replicate half was then subdivided in 1-cm-thick samples for analysis.
2 Recent Agricultural Occupation and Environmental Regeneration of Salt...