Temporal variation of delta C-13 in particulate organic matter and oyster Crassostrea gigas in Marennes-Oleron Bay (France): Effect of freshwater inflow

The temporal variability of delta(13)C in suspended particulate organic matter (POM) and oyster Crassostrea gigas along a salinity gradient was investigated from May 1992 to September 1993 within the estuarine bay of Marennes-Oleron (France). During this period the mean daily discharge of the Charente River exhibited large seasonal variation, with a high discharge from November 1992 to January 1993. Contrary to that at the river mouth and the marine littoral, delta(13)C in POM and in oysters at mid-estuary was affected by the high flood period. The delta(13)C values of POM decreased in mid-estuary and remained at low levels during the high discharge period, indicating an increasing contribution of terrestrial inputs to the estuarine POM pool. At the same site, a remarkable decrease of delta(13)C in oysters occurred between December 1992 and March 1993 (after a time lag compared to the ambient POM), indicating incorporation of terrestrial organic matter in oyster tissues during the high flood discharge. The lag between the delta(13)C decrease in POM and oysters is attributed to the time needed for oyster tissues to incorporate enough newly terrestrial light carbon to be recognized by the delta(13)C measure (about 1 to 2 mo). This time interval depends on tissue turnover time. The delta(13)C POM decrease (i.e. 1.3 parts per thousand) cannot explain entirely the decrease observed in oysters (i.e. 2.3 parts per thousand). In fact, the pattern exhibited by mid-estuarine oysters can be explained by the increasing contribution of terrestrial organic matter to their feeding, and the inability to preferentially utilize specific components of the estuarine POM that are C-13-enriched.

Age estimation in common sole Solea solea larvae: validation of daily increments and evaluation of a pattern recognition technique

Close similarities have been found between the otoliths of sea-caught and laboratory-reared larvae of the common sole Solea solea (L.), given appropriate temperatures and nourishment of the latter. But from hatching to mouth formation. and during metamorphosis, sole otoliths have proven difficult to read because the increments may be less regular and low contrast. In this study, the growth increments in otoliths of larvae reared at 12 degrees C were counted by light microscopy to test the hypothesis of daily deposition, with some results verified using scanning electron microscopy (SEM), and by image analysis in order to compare the reliability of the 2 methods in age estimation. Age was first estimated (in days posthatch) from light micrographs of whole mounted otoliths. Counts were initiated from the increment formed at the time of month opening (Day 4). The average incremental deposition rate was consistent with the daily hypothesis. However, the light-micrograph readings tended to underestimate the mean ages of the larvae. Errors were probably associated with the low-contrast increments: those deposited after the mouth formation during the transition to first feeding, and those deposited from the onset of eye migration (about 20 d posthatch) during metamorphosis. SEM failed to resolve these low-contrast areas accurately because of poor etching. A method using image analysis was applied to a subsample of micrograph-counted otoliths. The image analysis was supported by an algorithm of pattern recognition (Growth Demodulation Algorithm, GDA). On each otolith, the GDA method integrated the growth pattern of these larval otoliths to averaged data from different radial profiles, in order to demodulate the exponential trend of the signal before spectral analysis (Fast Fourier Transformation, FFT). This second method both allowed more precise designation of increments, particularly for low-contrast areas, and more accurate readings but increased error in mean age estimation. The variability is probably due to a still rough perception of otolith increments by the GDA method, counting being achieved through a theoretical exponential pattern and mean estimates being given by FFT. Although this error variability was greater than expected, the method provides for improvement in both speed and accuracy in otolith readings.