Greece
411
compressa, Gracilaria verrucosa and Punctaria latifolia are nowadays
encountered in these two zones. Scytosiphon lomentaria, which previously grew in this zone during the winter months, is now restricted to the
outer gulf. Thus the species number in the above two zones has been
slightly reduced, although diversity has never been large in the area.
The most conspicuous changes were observed in the sublittoral zone.
Within one decade, the number of macrophyte species was dramatically reduced from 127 to 30 by 1985 (Nikolaidis 1985). The decrease has
been both qualitative and quantitative. Total biomass decreased by
1000-1500 g m -2 (Haritonidis 1978; Haritonidis et al. 1986). The more
sensitive Phaeophyceae, previously forming the major portion of the
plant biomass, have nearly disappeared from the inner Thermaikos Gulf.
The species which are now dominant belong mainly to Chlorophyceae
(Haritonidis 1978; Haritonidis et al. 1986; Fig. 17.6). Before 1975, the
dominant species were Cystoseira fimbriata, Cystoseira barbata, Dictyopteris membranacea, Cladostephus verticillatus, Padina pavonica and
Taonia atomaria, but in 1985, Ulva rigida, Enteromorpha linza,
Enteromorpha compressa, Puncta ria latifolia, Scytosiphon lomentaria and
Codium spp. were dominant. Codium was seldom found in the Gulf up to
1975 (Haritonidis 1978). Since 1980, mass development of this seaweed
has occurred in both the inner and outer Thermaikos Gulf during the
winter months.
It is known that Codium strongly accumulates nitrate (Asare and
Harlin 1983; Luning 1990). Thus, the increase in nitrate in the Gulf may
possibly account for the explosive development of this seaweed. Most of
the thalli are comprised of Codium fragile with a diameter of 15-25 cm.
From 1967 on, since the beginning of seasonal investigations and
measurements Ulva rigida has been observed to gradually cover great
areas in the Gulf. At depths of 1-2 m, its biomass reached 10-12 kg m- 2
wet material. The biomass of Ulva displays seasonal variations with 5-6
kg m- 2 in spring, 10-12 kg m- 2 in summer, 6-7 kg m- 2 in autumn, and
2-3 kg m -2 in winter (Haritonidis et al. 1986).
The nitrophilous nature of Ulva and Enteromorpha was also checked
under experimental conditions using a medium of sewage and seawater
in which 95% of nitrate and other nutrients were absorbed within 2 - 3
days (Haritonidis et al. 1991). The nitrophilous characters of the species
were also verified by the tremendous increase in the Ulva sp. biomass
which grows in enriched seawater medium with a 15:1 ratio of N:P.
The highest relative growth - up to 95% weight increase per experimental period (10 days) - was observed in culture when nitrogen was
mainly supplied in the form of NH 4 -N (Fig. 17.7; Kotropoulos et al.
1991).
411
compressa, Gracilaria verrucosa and Punctaria latifolia are nowadays
encountered in these two zones. Scytosiphon lomentaria, which previously grew in this zone during the winter months, is now restricted to the
outer gulf. Thus the species number in the above two zones has been
slightly reduced, although diversity has never been large in the area.
The most conspicuous changes were observed in the sublittoral zone.
Within one decade, the number of macrophyte species was dramatically reduced from 127 to 30 by 1985 (Nikolaidis 1985). The decrease has
been both qualitative and quantitative. Total biomass decreased by
1000-1500 g m -2 (Haritonidis 1978; Haritonidis et al. 1986). The more
sensitive Phaeophyceae, previously forming the major portion of the
plant biomass, have nearly disappeared from the inner Thermaikos Gulf.
The species which are now dominant belong mainly to Chlorophyceae
(Haritonidis 1978; Haritonidis et al. 1986; Fig. 17.6). Before 1975, the
dominant species were Cystoseira fimbriata, Cystoseira barbata, Dictyopteris membranacea, Cladostephus verticillatus, Padina pavonica and
Taonia atomaria, but in 1985, Ulva rigida, Enteromorpha linza,
Enteromorpha compressa, Puncta ria latifolia, Scytosiphon lomentaria and
Codium spp. were dominant. Codium was seldom found in the Gulf up to
1975 (Haritonidis 1978). Since 1980, mass development of this seaweed
has occurred in both the inner and outer Thermaikos Gulf during the
winter months.
It is known that Codium strongly accumulates nitrate (Asare and
Harlin 1983; Luning 1990). Thus, the increase in nitrate in the Gulf may
possibly account for the explosive development of this seaweed. Most of
the thalli are comprised of Codium fragile with a diameter of 15-25 cm.
From 1967 on, since the beginning of seasonal investigations and
measurements Ulva rigida has been observed to gradually cover great
areas in the Gulf. At depths of 1-2 m, its biomass reached 10-12 kg m- 2
wet material. The biomass of Ulva displays seasonal variations with 5-6
kg m- 2 in spring, 10-12 kg m- 2 in summer, 6-7 kg m- 2 in autumn, and
2-3 kg m -2 in winter (Haritonidis et al. 1986).
The nitrophilous nature of Ulva and Enteromorpha was also checked
under experimental conditions using a medium of sewage and seawater
in which 95% of nitrate and other nutrients were absorbed within 2 - 3
days (Haritonidis et al. 1991). The nitrophilous characters of the species
were also verified by the tremendous increase in the Ulva sp. biomass
which grows in enriched seawater medium with a 15:1 ratio of N:P.
The highest relative growth - up to 95% weight increase per experimental period (10 days) - was observed in culture when nitrogen was
mainly supplied in the form of NH 4 -N (Fig. 17.7; Kotropoulos et al.
1991).