Proceedings of the Second Mediterranean Symposium on Marine

PROCEEDINGS OF THE SECOND MEDITERRANEAN SYMPOSIUM ON MARINE VEGETATION (ATHENS, 12-13 DECEMBER 2003)
238
SEAGRASS ECOSYSTEMS AS BIOLOGICAL INDICATORS.
A COMPARISON OF TWO APPROACHES: LEAF EPIPHYTE TAXONOMY
AND A COMBINED SET OF BIOLOGICAL DESCRIPTORS
Begoña MARTINEZ, Adriana VERGÉS TRAMULLAS, Patricia PRADO, Javier ROMERO and
Teresa ALCOVERRO.
ABSTRACT
The main objective <strong>of</strong> <strong>the</strong> present study is <strong>the</strong> comparison <strong>of</strong> two separate approaches
to <strong>the</strong> use <strong>of</strong> P. oceanica meadows as biological indicators <strong>of</strong> water quality. One is based
on <strong>the</strong> use <strong>of</strong> a combined set <strong>of</strong> biological descriptors (physiological, morphological,
structural, communal and environmental) and <strong>the</strong> o<strong>the</strong>r is based on <strong>the</strong> taxonomy <strong>of</strong> leaf
epiphytes. Both sets <strong>of</strong> descriptors were measured in 12 locations, eight <strong>of</strong> which were
deep (15 m) and four <strong>of</strong> which were shallow (5 m). The selection <strong>of</strong> <strong>the</strong>se locations was
based on a previous four-year seagrass monitoring study and was aimed at <strong>the</strong>
homogenous cover <strong>of</strong> a wide range <strong>of</strong> water qualities, from poor to quasi-pristine
conditions. Principal component analysis (PCA) applied to a set <strong>of</strong> biological descriptors
resulted in two principal components which explained 46% <strong>of</strong> <strong>the</strong> variance. Component
I appears to be related to conditions <strong>of</strong> eutrophia and metallic contamination and
explains 28.8% <strong>of</strong> variance. Component II explains 17.2% <strong>of</strong> variance and seems to be
related to herbivory, which is considerably more intense in shallow locations.
Correspondance analysis applied to leaf epiphyte taxonomy <strong>of</strong> axis I and II explain 22%
and 17.1% <strong>of</strong> <strong>the</strong> variance respectively. Axis I provide an ordering <strong>of</strong> locations that is
related to existing water quality data, whilst axis II again seems to be related to herbivory.
The two approaches are coherent with one ano<strong>the</strong>r, and <strong>the</strong>y both provide a good
indication <strong>of</strong> water quality. The ordering <strong>of</strong> locations provided by both sets <strong>of</strong> descriptors
is coherent with existing water quality data from <strong>the</strong> region. Even though <strong>the</strong> two
approaches provide remarkably similar results, <strong>the</strong> combined set <strong>of</strong> biological indicators
<strong>of</strong> <strong>the</strong> Posidonia oceanica ecosystem appears as a more powerful tool in <strong>the</strong>
discrimination <strong>of</strong> medium-to-good water qualities. Meadows found in shallow waters are
clearly influenced by internal forcings such as herbivory, which are in no apparent way
related to water quality. For that reason, deeper meadows are recommended for use in
any future monitoring studies.