Analisi della produzione primaria in Posidonia oceanica
Author(s)
Dolce, Tania
Date Issued
March 15, 2010
Type
Doctoral Thesis
Abstract
Posidonia oceanica meadows respond strongly to both natural and anthropogenic alterations in
resources availability, and are able to record changes of environmental variables such as light,
nutrients or metal concentrations. This ability of the system to record changes of environmental
quality put it forward as an excellent bioindicator in monitoring studies (Pergent et al., 1999).
Biomonitoring is usually applied at a regional level, where multiple impacts can act simultaneously and interact with natural variation at different spatial scales. Yet the choice of variables to measure and the sampling strategy are critical issues for the validity of the results.
The aim of this work is to examine the method to estimate primary production of Posidonia, to
improve basic knowledge and to focus on critical use of this descriptor. The attention is mainly on
lepidochronological analysis, a reconstructive technique for the identification of yearly cyclic leaf production along the Posidonia oceanica rhizomes. Moreover, it is possible to backdate every single rhizome and to estimate mean growth represented by extention and biomass of the rhizome and by number of leaves formed during an annual cycle. This thecnique has been recommended as tool of analysis of seagrass dynamics, including annual growth and production.
Furthermore the study presents the result of a screening process on a set of biological descriptors obtained from the P. oceanica ecosystem (morphological and structural), aiming at:
(i) evaluating their response to some environmental factors (such as depth, substratum,
hydrodynamic level, anthropogenic impacts) and, as a result, their value and adequacy as indicators;
(ii) selecting an optimal subset of indicators better reflecting the response of the system to
environmental factors.
Sets of descriptors were measured in 700 locations, separate in deep (>15 m), intermediate and shallow sites (<10 m).
The study allowed to correct the reconstructive technique and to obtain thorough assessment of
primary production. Leaf production assessment seems to be more accurate using reduced mayor
axis than regression line to estimate the positive correlation between blade and sheath lenght. The spatial and temporal dynamics of the sheath and blade relationship had to be taken into account during each sampling to obtain the mean annual production.
Furthermore, analysis on rhizome production highlighted an increase of dry weight in the first 3-4 years and a decrease of dry weight per unit lenght in the cicles following the tenth; it allows to correct average value of rhizome production.
The Pearson’s and Spearman’s correlation coefficients were used to investigate the relationships between primary production of both above-ground and below-ground tissues and selected variables.
Statistical analyses showed that above- and below-ground production change in relation to substrata
type (sand, rock, matte) and also at meadow level, shoot density shows again differences among
three substrata. The below-ground production seems to be closely related to absolute density and to hidrodinamic level, whereas leaf surface results positive correlated to impact level.
We hypothesize that shoots living under stressed conditions enlarge the leaf surface to optimize
photosynthetic yield and that vertical growth rate closely reflects density variation, which suggests that seagrass may be sensitive sensor of changes of the sediment level.
This research points out that at individual level the primary production can be considered a good
effective descriptor able to show different roles carried on by rhizome and leaf shoot and their
different relationships with environmental variables.
The relationship between above- and below- ground production highlighted the rhizome capacity to
store resources, allowing the plant to support growth patterns comparatively independent from
environmental conditions; the results confirm that variation of environmental forcing in P. oceanica is probably buffered by the availability of internal resources stored in the below-ground
compartment.
In order to evaluate the descriptors showing significant variability between sites and to obtain a two-dimensional representation of our sites, a principal component analysis (PCA) was performed including both shallow and deep meadows.
Significance of different descriptors and their relative contribution to spatial variability largely differ among sampling depths.
Concerning the recommended use of Posidonia oceanica as an indicator of changed environmental
quality in relation to several human-induced stresses, caution should be used when interpreting
responses measured at different complexity levels, as our data indicates that they may be
inconsistently different and cannot be, therefore, generalised.
Posidonia oceanica responses to variation of environmental factors are integrated, so a combination of different descriptors would be recommended in order to unveil at which level specific factors may affect its ecosystem.
Additional information
Dottorato di ricerca in Ecologia e gestione delle risorse biologiche
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