Abstract |
The size of an organism is a “master trait” governing multiple biological and ecological
processes. The metabolic rate, the growth rate of the body and body parts, the growth rate of a
population, the mortality rate, the maximum lifespan, several reproductive traits, predator-prey
relationships and the size-structure of organisms, are few of the processes dependent on the size of
the organism. Through the normalized biomass size-spectrum (NBSS) the efficiency of the energy
flow from the smaller to the larger organisms can be estimated. The slope of the spectrum reflects the
metabolic losses between trophic levels and the losses to the detritus food chain.
In the present study, the main factors (biotic and abiotic) influencing the size-spectra of aquatic
organisms were investigated. Towards this objective, three case-studies were carried out in three areas
of the Aegean Sea, with different physical, hydrological and anthropogenic disturbance
characteristics. An extensive review on the published effects on aquatic size-spectra was also carried
out.
The three areas considered were: a) The broader area of the Saronic Gulf, in which Elefsina Bay
is located. Elefsina Bay is a highly impacted enclosed gulf, that exhibits near bottom hypoxia during
the summer stratification period. This area was chosen to investigate the effect of hypoxia on the
biomass size-spectra of planktonic organisms. b) The Thracian Sea, an area of intense hydrological
variability, highly affected by the advection of low salinity and productive Black Sea Waters. The
small-scale and depth related effects of said waters on the biomass size-spectra of mesozooplankton
was explored. c) The Cretan Sea, which is one of the most oligotrophic areas in the Mediterranean
Sea, characterized by the absence of anthropogenic disturbances and high hydrological variability
due to the mixing of different water masses coming from the Levantine, the Atlantic Ocean and the
Black Sea. Said area was chosen to investigate the stability of the biomass size-spectrum, presented
generally in oligotrophic oceans, and the seasonal and interannual variability of the size-spectrum
during a six-year period.
We found that near-bottom hypoxia during the stratification period in Elefsina Bay severely
impacted the slopes and y-intercepts of the biomass size-spectra of planktonic organisms. This was
accompanied by concomitant shifts in the distributions of organisms throughout the water column,
wherein the picoautotrophic eukaryotes dominated in the hypoxic layer, while mesozooplankton was
actively avoiding it. The present study quantifies for the first time the effects of hypoxia on the
biomass size-spectra of pelagic planktonic organisms. The results indicate that hypoxia reduces
trophic transfer efficiency from the smaller to the larger organisms.
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The productive, low salinity Black Sea Water intrusions led to the formation of strong fronts in
the Thracian Sea, heavily influencing the biomass size-spectra and the distributions of
mesozooplanktonic organisms. Stations located on the fronts formed by the movement of the main
current presented flatter slopes and higher y-intercepts compared to the closely located (~5 NM)
adjacent stations. The productive Black Sea Waters, known to carry dissolved and particulate organic
carbon, enhance secondary production and increase mesozooplankton biomass. Depth related effects
on the size-spectra and organism distributions were also observed. Slopes were flatter and y-intercepts
higher directly above the halocline, wherein the highest mesozooplankton biomasses were recorded.
Such a high, small-scale, spatial variability of the size-spectrum parameters is, to our knowledge,
demonstrated for the first time. How systems of high hydrological variability evolve through time, is
also discussed.
Remarkable stability of the biomass size-spectra through time was observed in the Cretan Sea.
Both slopes and y-intercepts exhibited very small variability, without any seasonal or interannual
pattern. This remarkable stability suggests that, in the hyper-oligotrophic area of the Cretan Sea, the
organization of the planktonic trophic web, and especially the contribution of the recycling of the
organic matter through the microbial food web, supports a high trophic transfer efficiency throughout
the year. The time-series of the Cretan Sea is one of the few published multiyear, with monthly
resolution, size-spectra time-series.
A multitude of factors influencing the aquatic size-spectra was recorded through extensive
review of publicized information. The factors identified refer to the general shape of the spectrum,
the physical, topographical and oceanographic variability, the season, the concentration of nutrients,
the biological and ecological variability, and the anthropogenic disturbances, including fisheries and
climate change. The organism size-spectra are proven to be powerful tools for the assessment and
monitoring of the state of aquatic ecosystems.
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