404
13 Transport and Mixing in Coastal Ecosystems
in estuaries. Some portion of available phosphorus is dissolved and is available for uptake by planktonic algae. The inorganic species of nitrogen, such
as ammonium (NH4) and nitrate (N03), are rapidly taken up and mineralized
by plankton. Organic matter is mineralized in the water column and the benthos. Half of the organic nitrogen which falls to the bed in estuaries may be
removed from the ecosystem by de nitrifying bacteria (Furnas, 1996; Alongi,
1998). When estimating net fluxes of nutrients, pollutants or other compounds
in coastal marine ecosystems, all these processes have to be taken into account.
Muller et at. (1994) examined dissolved, leachable particulate trace elements
(Fe, Mn, Zn, Pb, Cd, Co, Cu), particulate organic carbon and dissolved nutrients (P04, NH3 , N03, N02, Si) in the Clyde Estuary, Scotland. They found
that the transport of metals to the coastal waters of the estuary takes place
predominantly in the suspended solid phase for Fe and Pb, in the dissolved
phase for Cd, and with comparable contributions from both phases for Mn,
Zn, Co, Ni and Cu. The degree of enrichment increases towards the mouth of
estuary and is dependent on the residence time (always shorter than 3-7 days)
and freshwater input.
In Sect. 8.5.1 we have examined various types of estuaries. In particular,
in shallow and narrow estuaries dominated by high river discharge, a saltwedge occurs. The velocity shear between the freshwater and salt water flows
induces an entrainment process. For the upward entrainment in estuaries,
salinity increases in the upper layer. The dynamics and mixing of nutrients
is a function of the entrainment mechanisms and the concentration of source
nutrients. When a river has a lower nutrient concentration than the ocean
water, entrainment results in an increase of nutrient concentration in the surface
layer. However, when there is higher nutrient concentration in the river, the
entrainment of the sea water results in dilution. The entrainment mechanism,
which is supposed to play an important role in nutrient supply, was examined
by Yin et at. (1995a, b, c) using data collected in the Fraser River estuary
(British Columbia, Canada). The Fraser River is one of the largest rivers along
the west coast of North America. River discharge reaches its maximum of 10
000 m 3 /s in June, and gradually decreases to about 700 m 3 /s in September,
remaining at that level until March, when it starts to increase again. The river
discharge is modulated by tides, when a salt wedge invades the river during
tidal flood and retreats during tidal ebbs.
According to Yin et at. (1995a), in the Fraser River estuary there are
three water masses: the riverine plume, the estuarine plume, and deep water
(Fig. 13.4). The mixing of nitrate, N03, depends on tides and river discharge.
During a flood tide, the salt wedge invades the river at depth and the discharge
of freshwater decreases. The estuarine plume is pushed into the river at the
surface (Fig. 13.4a). Little upward mixing takes place during the advance of
the salt wedge. During a tidal ebb, when the salt wedge starts to retreat, intensive entrainment takes place between both layers, resulting in broadening of
the halo cline. After the salt wedge is broken down and swept out of the river,
13 Transport and Mixing in Coastal Ecosystems
in estuaries. Some portion of available phosphorus is dissolved and is available for uptake by planktonic algae. The inorganic species of nitrogen, such
as ammonium (NH4) and nitrate (N03), are rapidly taken up and mineralized
by plankton. Organic matter is mineralized in the water column and the benthos. Half of the organic nitrogen which falls to the bed in estuaries may be
removed from the ecosystem by de nitrifying bacteria (Furnas, 1996; Alongi,
1998). When estimating net fluxes of nutrients, pollutants or other compounds
in coastal marine ecosystems, all these processes have to be taken into account.
Muller et at. (1994) examined dissolved, leachable particulate trace elements
(Fe, Mn, Zn, Pb, Cd, Co, Cu), particulate organic carbon and dissolved nutrients (P04, NH3 , N03, N02, Si) in the Clyde Estuary, Scotland. They found
that the transport of metals to the coastal waters of the estuary takes place
predominantly in the suspended solid phase for Fe and Pb, in the dissolved
phase for Cd, and with comparable contributions from both phases for Mn,
Zn, Co, Ni and Cu. The degree of enrichment increases towards the mouth of
estuary and is dependent on the residence time (always shorter than 3-7 days)
and freshwater input.
In Sect. 8.5.1 we have examined various types of estuaries. In particular,
in shallow and narrow estuaries dominated by high river discharge, a saltwedge occurs. The velocity shear between the freshwater and salt water flows
induces an entrainment process. For the upward entrainment in estuaries,
salinity increases in the upper layer. The dynamics and mixing of nutrients
is a function of the entrainment mechanisms and the concentration of source
nutrients. When a river has a lower nutrient concentration than the ocean
water, entrainment results in an increase of nutrient concentration in the surface
layer. However, when there is higher nutrient concentration in the river, the
entrainment of the sea water results in dilution. The entrainment mechanism,
which is supposed to play an important role in nutrient supply, was examined
by Yin et at. (1995a, b, c) using data collected in the Fraser River estuary
(British Columbia, Canada). The Fraser River is one of the largest rivers along
the west coast of North America. River discharge reaches its maximum of 10
000 m 3 /s in June, and gradually decreases to about 700 m 3 /s in September,
remaining at that level until March, when it starts to increase again. The river
discharge is modulated by tides, when a salt wedge invades the river during
tidal flood and retreats during tidal ebbs.
According to Yin et at. (1995a), in the Fraser River estuary there are
three water masses: the riverine plume, the estuarine plume, and deep water
(Fig. 13.4). The mixing of nitrate, N03, depends on tides and river discharge.
During a flood tide, the salt wedge invades the river at depth and the discharge
of freshwater decreases. The estuarine plume is pushed into the river at the
surface (Fig. 13.4a). Little upward mixing takes place during the advance of
the salt wedge. During a tidal ebb, when the salt wedge starts to retreat, intensive entrainment takes place between both layers, resulting in broadening of
the halo cline. After the salt wedge is broken down and swept out of the river,