Biomarkers
contribution, as determined by long-chain satured FA (C26:0-C32:0)
proportions, is low. The corresponding particules are settling rapidly
and are observed principally at or near the bottom during flood or ebb
resuspensions.
In copepods, diatom FAs, such as C20:5 ω3, C16:2 ω4, C16:3 ω4 and
also C16:1 ω7 are found (table 1). The C18:3 ω3 is also found but in
minor amounts. Besides high proportions of cholesterol, low proportions of diatoms sterols such as trans-22 deshydrocholesterol and brassicasterol are also found in copepods (table 2). Campesterol and even
part of cholesterol could also been of diatom origin. These two sterols
are the predominant compounds synthetized by Melosira granulata in
which they represent 21% (campesterol) and 67% (cholesterol) of total
sterols (Nishimura & Koyama, 1977). The low proportions of branched
FAs, and the rather low value of C18:1 ω7/C18:1 ω9 ratio, seem to
indicate that bacteria are not a significant food source for the copepods.
Detritic particules from terrestrial plants and from sewage are not
ingested by the copepods because ol the absence of long-chain FAs ancl
the absence of coprostanol and 24-ethylcoprostanol. Phytoplankton,
mainly diatoms, appears here as the predominant food source for Eurytemora affinis.
Shrimp stomach composition is globally similar to copepod composition
(tables 3 and 4). However, some specific features are noticed. Comparatively to copepods, shrimp stomachs are richer in the two essential
FAs, C20:5 ω3 and C22:6 ω3. On the contrary, the proportions of other
low interesting FAs, from a nutritional point of vue, such as C16 and
08 polyunsaturated FAs are lower. Among sterols, cholesterol and
cholestanol are greatly predominant, while phytosterols are in low proportions. All these results indicate that shrimps feed principally on
copepods in the Seine estuary, at least at the time of our study. As
copepods, shrimps do not ingest particles from terrestrial plants or from
sewage. Bacteria do not seem to be a significant food source for them.
In shrimp faecal pellets, the less interesting FAs and STs, from a nutritional point of view, are found in higher proportions than in stomachs.
This is the case for saturated FAs, for branched FAs and for phytosterols (tables 3, 4). The essential C20:5 ω3 and C22:6 ω3 are also found
in pellets, as well as cholesterol. This means that some losses of these
products occur during ingestion. However, these losses are very low,
because in absolute quantities, the FAs and STs present in faecal pellets
at the end of incubation represent a few percentage of FAs and STs
present in stomachs before incubation (data not shown).
Conclusion
At the time of our study, evidence was given that in a higly detritic
environment, such as the Seine estuary, the copepod Eurytmora affinh
feeds principally on phytoplancton. The shrimp Palaemon longirostris leeds
principally on Eurytemora affinis. Other potential food sources, such as
85
contribution, as determined by long-chain satured FA (C26:0-C32:0)
proportions, is low. The corresponding particules are settling rapidly
and are observed principally at or near the bottom during flood or ebb
resuspensions.
In copepods, diatom FAs, such as C20:5 ω3, C16:2 ω4, C16:3 ω4 and
also C16:1 ω7 are found (table 1). The C18:3 ω3 is also found but in
minor amounts. Besides high proportions of cholesterol, low proportions of diatoms sterols such as trans-22 deshydrocholesterol and brassicasterol are also found in copepods (table 2). Campesterol and even
part of cholesterol could also been of diatom origin. These two sterols
are the predominant compounds synthetized by Melosira granulata in
which they represent 21% (campesterol) and 67% (cholesterol) of total
sterols (Nishimura & Koyama, 1977). The low proportions of branched
FAs, and the rather low value of C18:1 ω7/C18:1 ω9 ratio, seem to
indicate that bacteria are not a significant food source for the copepods.
Detritic particules from terrestrial plants and from sewage are not
ingested by the copepods because ol the absence of long-chain FAs ancl
the absence of coprostanol and 24-ethylcoprostanol. Phytoplankton,
mainly diatoms, appears here as the predominant food source for Eurytemora affinis.
Shrimp stomach composition is globally similar to copepod composition
(tables 3 and 4). However, some specific features are noticed. Comparatively to copepods, shrimp stomachs are richer in the two essential
FAs, C20:5 ω3 and C22:6 ω3. On the contrary, the proportions of other
low interesting FAs, from a nutritional point of vue, such as C16 and
08 polyunsaturated FAs are lower. Among sterols, cholesterol and
cholestanol are greatly predominant, while phytosterols are in low proportions. All these results indicate that shrimps feed principally on
copepods in the Seine estuary, at least at the time of our study. As
copepods, shrimps do not ingest particles from terrestrial plants or from
sewage. Bacteria do not seem to be a significant food source for them.
In shrimp faecal pellets, the less interesting FAs and STs, from a nutritional point of view, are found in higher proportions than in stomachs.
This is the case for saturated FAs, for branched FAs and for phytosterols (tables 3, 4). The essential C20:5 ω3 and C22:6 ω3 are also found
in pellets, as well as cholesterol. This means that some losses of these
products occur during ingestion. However, these losses are very low,
because in absolute quantities, the FAs and STs present in faecal pellets
at the end of incubation represent a few percentage of FAs and STs
present in stomachs before incubation (data not shown).
Conclusion
At the time of our study, evidence was given that in a higly detritic
environment, such as the Seine estuary, the copepod Eurytmora affinh
feeds principally on phytoplancton. The shrimp Palaemon longirostris leeds
principally on Eurytemora affinis. Other potential food sources, such as
85