|Action A4. Preliminary study of the demography of Posidonia fields|
Aims: The preliminary study was to gather and compile information on the stability, demography and foreseeable evolution (stationary, in recession, or in progression) of the habitat within the reserve, as well as to identify and define the original initial state in order to later on be able to follow the effect that establishing a reserve produces on the fields.
Planning and Development:
During 2000 and 2002, three permanent plots were marked,
these being of between 0.09m2 and 0.72m2, and bushes were physically marked
off at 35 fields round the Balearic Islands, 77 % of which were in SCI
areas, plus also 9 more in fields pertaining to the Spanish mainland coast.
Scuba-diving census of the Posidonia oceanica bushes in the fields situated at Es Castell, off the island of Cabrera.
On the one hand, comparison of the demographic balance
of bushes in the fields studied at present (2000-2002) with that of the
last 3 decades, revealed that the state of
Table 1. State of the fields of P. oceanica in SCI areas of the Balearics at present (between 2000 and 2002), and during the last 3 decades. “nd” indicates there are no available estimates. This “nd” lack of estimates on the present state of P.oceanica is due to either the loss of marked plots or to the fact that during the period of time comprised by the Action there were some instances in which it had only been possible to mark off the plots without there being enough time to carry out a proper bush census.
Application of the results obtained through Action A.4. to the project.
The results obtained provided, for the first time, an
evaluation of the state of Posidonia oceanica in SCI areas round the Balearics.
This study provided a basis from which to later evaluate the effect of
the measures which implemented in order to help maintain the state of
P.oceánica’s health in SCI areas of the Balearics. Therefore
the results of this Action provided the initial working data for Action
F3 of the project.
1.-Bay of Pollensa SCI: The field of P.oceánica in this area is in strong recession. The net growth rate of the population indicated that, if present environmental conditions continue, the density of the bushes in this field will shrink by half in just 5 years. Besides , the density of the bushes was found to be low, considering the depth at which the studied P.oceánica grows, and this fact indicated that perhaps the decline was not recent. The presence of Caulerpa prolifera in this field suggested the existence of a lack of oxygen and intense bacterial activity feeding off the supply of organic matter found in the sediment upon which P.oceánica grows, and these environmental conditions may be the cause of the recession detected in this field of P.oceanica.
2. Bay of Alcudia SCI : The demographic balance at the study station indicated that the field of P. oceanica was in a stationary state. The good health of the P.oceanica was reflected also in the density values of the bushes it held (normal). Nevertheless the presence of C.prolifera in the field indicates that in the near future the quality of the sediment could deteriorate, and health damaging environmental conditions could develop that would reduce the persistence of the P. oceanica field.
3. East Coast SCI: The field of P.oceánica in this area is in recession. The net growth rate of the population indicated that if present environmental conditions persist, the density of the bushes will be reduced by half in just 6.3 years. Besides , the density of bushes was found to be low, considering the depth at which the studied P.oceánica grows, and this fact indicated that perhaps the decline was not recent.
4. Cap Enderrocat-Cap Blanc SCI: The density of bushes in the studied P.oceánica field was somewhat low when compared with the density of bushes found in other P.oceánica fields situated at similar depths round the Balearics. This suggestes that the environmental conditions existing in this area could be negatively altering the field of P.oceanica. Nevertheless, the lack of demographic estimates did not allow us to accurately establish the state of the P.oceanica field in this area.
5. Cap Salines SCI : The P.oceánica field situated right off the Colonia San Jordi shore was on average in a stationary phase. Nevertheless intense heterogeneous variations were detected in the demographic dynamics of P.oceanica for there were large swathes of dying bushes situated right next to swathes where the bushes were in expansion. The density of bushes at the Colonia San Jordi field was high, which indicated that the field was in good health. The field developing off Cap Salinas was in a state of slight recession. If the present deteriorating environmental conditions persist, the density of the bushes in this field will be reduced by half in about 8 years. The recession detected in this field did, however, seem to have begun recently, as the density of bushes was relatively high in comparison with other fields growing at similar depths round the Balearics.
6. Cabrera SCI: The fields studied off Cabrera, a national park with restricted access for visitors and for anchored yachts since 1993, tend to recede. In some areas of the Bay d’El Port, however, P.oceanica presented stable dynamics. The general tendency towards recession detected in both bays indicated the presence of adverse environmental factors deriving from both human pressure and natural disturbances which deteriorate the health of the fields. If present environmental conditions persist, fields situated off El Port will reduce their density by half in a period of time of between 2.3 and 3.8 years, whereas the field off Santa María would take about between 3.5 and 7.3 years to reduce its density of bushes. In spite of the general tendency towards recession found in the population of P.oceánica in this SCI area, the difference in density of bushes in the fields studied at these two bays suggests that the decline in the Santa María field began recently whereas the decline of the field situated at El Port has been in process for a number of years.
7. Tagomago SCI: The field of P.oceánica in this SCI enjoyed good health, as witnessed by a high density of bushes and a positive demographic balance due to the very low death rate suffered by this field. The results obtained showed that this field was in expansion. If present environmental conditions persist, the growth rate of this population indicates that density could double in 7.7 years. The rate of expansion observed, however, will be able to continue only on the outer fringes of the field as density of the clones in the inner parts of the field was already at a maximum.
8. Ses Salines of Ibiza-Formentera SCI. The P.oceánica fields of the Ibiza-Formentera SCI area were in good health and in a stationary state off Cala Torreta, tending to expand off Illetas, and in slight recession off Els Pujols. Pressure exerted by anchoring yachts in the whereabouts of Es Caló de s’Oli may deteriorate the state of P.oceanica in the area.
9. Marina of the North of Minorca SCI area: The estimations on the density of bushes in this field is among the top high-density fields of P.oceanica, considering the depth at which it grows, and indicates that this field enjoys good health.
10. D'Addaia at s'Albufera SCI: The density of bushes in the field studied at this SCI area is high in comparison with the density of bushes found in other fields colonizing similar depths, and this fact indicates that the field is most probably in good health.
The results, therefore , identify an important number of fields in recession (SCI areas of Cabrera, Ses Salines of Ibiza-Formentera, Bay of Pollensa, East coast and Cap Salines), upon which urgent action is called for in order to prevent deterioration from continuing. The necessary measures to be implemented in order to alleviate the pressure suffered by these fields and bolster their recovery and conservation, are defined in other parts of the SCI project and include such measures as regulating yacht anchoring as well as the design of management plans. In particular, it has been suggested that these fields should be the first to be protected by regulations governing human activities such as yacht anchoring, and these measures have in effect been taken. Furthermore, efforts should be made to monitor these fields by encouraging the creation of monitoring groups within the existing Monitor Network in charge of monitoring P.oceanica fields in the Balearics and which is dependant of the General Directorate of Fishing (DGP). Since 2002, the DGP has defined and listed the points which should be checked when monitoring the state of P.oceanica at these SCI areas, as well as in recently created SCI areas, and in non-SCI areas round the Balearics. At the same time work is being undertaken to identify the most important causes of recession of P.oceanica fields, because obviously,
conservation measures can only be effective if the causes of damage are known and for this reason defining said causes is vital. Results obtained from this action have, therefore, provided the essential information that should be taken into account when developing management plans and implementing measures regulating yacht anchoring in SCI areas round the Balearics.