STRIVE - Environmental Protection Agency

3.3.3 Relationships between Chlorophyll-a,
Temperature and Secchi Depth
Chlorophyll-a concentrations greater than 5 µg.l-1 were
generally reached when temperatures were near 18ºC
(Fig. 3.6). Higher chlorophyll-a levels were associated
with low Secchi depths, in particular between weeks
5 and 10. The low chlorophyll-a concentrations
observed at the beginning of the study coincided with
high water clarity, whereas the lakes sampled near the
end of the summer showed both low phytoplankton
biomass and weak light penetration, suggesting the
presence of humic substances or perhaps heavy
loads of suspended matter in the water column
concomitant to bad weather and rainfall.
N. Touzet (2008-FS-EH-3-S5)
Figure 3.6. Scatter plots between chlorophyll-a and temperature (a), and between chlorophyll-a and Secchi
depth (b) in the lakes sampled during summer 2009.
3.3.4 Trophic Status of Lakes and Associated
Land Cover Type
Contingency analysis was carried out to determine
the presence of significant relationships between
the nature of land cover and the trophic state of the
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waterbodies sampled in the study area (Fig. 3.7).
The analysis was performed using four trophic state
classes (ultra-oligotrophic, oligotrophic, mesotrophic
and eutrophic) and the three geographical zonation
groups defined following examination of the indices
data. When feasible, publicly available water body
categorisation, previously carried out by the Irish
EPA, was used for the lakes sampled (EPA Ireland,
2008b). The trophic state of the remaining lakes was
estimated using the chlorophyll-a concentrations
and Secchi depth values determined in this study
according to the eutrophication classification levels
defined in OECD (1982). The contingency analysis
showed a significant relationship between trophic
status and the zonation groups (X = 17.35, p < 0.05,
df = 6). A further significant relationship was found in
relation to spatial distribution between trophic status
and the clusters of river catchment (excluding lakes
from clusters C and F) (X = 20.22, p < 0.05, df = 9,
n = 46).