Dg. George et Ij. Winfield, Factors influencing the spatial distribution of zooplankton and fish in Loch Ness, UK, FRESHW BIOL, 43(4), 2000, pp. 557-570
1. The vertical and horizontal distribution of phytoplankton, zooplankton a
nd fish in Loch Ness, Scotland, were monitored during one day-time and one
night-time survey in July 1992. The vertical samples were collected at a si
te located at the northern end of the loch and the horizontal samples along
a longitudinal transect.
2. The vertical distribution surveys demonstrated that the phytoplankton, t
he zooplankton and the fish were concentrated in the top 30 m of water abov
e the seasonal thermocline. Within this layer, Cyclops stayed much closer t
o the surface than Eudiaptomus but both species moved towards the surface a
t night.
3. The most important factor influencing the horizontal distribution of the
phytoplankton was the north-south gradient in productivity. The sub-catchm
ents surrounding the north basin contain a greater proportion of arable lan
d than those to the south and the concentrations of nitrate-nitrogen and ph
ytoplankton chlorophyll increased systematically from south to north.
4. Zooplankton distribution patterns were influenced by wind-induced water
movements and the dispersion of allochthonous material from the main inflow
s. The highest concentrations of Cyclops were recorded in the north, where
there was more phytoplankton, and the highest concentrations of Eudiaptomus
in the south, where there were higher concentrations of non-algal particul
ates.
5. There was no spatial correlation between total zooplankton and total fis
h abundance but the highest concentrations of small (1-5 cm) fish were reco
rded in the south where there was a large patch of Eudiaptomus. The number
of Eudiaptomus at specific locations within this patch were, however, negat
ively correlated with the numbers of small fish. These results suggest that
the fish were actively foraging within the patch and were depleting their
zooplankton prey in the areas where they were most abundant.