Size spectra exhibit common patterns of variation and predictable responses to pressures across ecosystem
types, functional guilds and taxonomic groups. Here, we extend the size spectra approach to
phytoplankton ecological status assessment in transitional waters by developing, testing and validating
a multi-metric index of size spectra sensitivity (ISS-phyto), which integrates size structure metrics
with others such as phytoplankton diversity, biomass and sensitivity of size classes to anthropogenic
disturbance. The ability of various theoretical models of size spectra sensitivity to discriminate between
disturbed and undisturbed ecosystems and levels of anthropogenic stress was evaluated. We used data
on phytoplankton samples collected in 14 Mediterranean and Black sea transitional water ecosystems
(coastal lagoons) from Italy, Albania, Greece, Bulgaria and Romania, and compared the models’ efficiency
by looking at their pressure–impact response along salinity and enrichment gradients, the latter quantified
as variations in dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP). Data
from a fifteenth Mediterranean lagoon was used for external validation purposes. Right asymmetric models
of size class sensitivity, implying higher sensitivity of smaller cell size classes, were found to contribute
to the ISS-phyto multimetric tool more effectively than symmetric and left asymmetric models, distinguishing
disturbed from undisturbed lagoons and disturbed from undisturbed stations within the same
lagoon. When based on right asymmetric sensitivity models, i.e., those that were most efficient in identifying
anthropogenic impacts, ISS-phyto also showed the best fit of pressure–response relationships along
the salinity and enrichment gradients; at low to high levels of impact ISS response was driven by size
class sensitivity and at very high impacts by phytoplankton biomass. A scheme for the classification of
Ecological Quality Status based on ISS-phyto is proposed and validated. The validation procedure found
that ISS-phyto is an effective and sensitive monitoring tool, robust, easy to apply and to inter-calibrate
among laboratories.
