Bivalve diets in a midwestern U.S. stream: A stable isotope enrichment study

Abstract

This study examined a community of stream bivalves (unionids and fingernail clams) in a second-order woodland stream in southern Michigan using both the natural abundance of 15 N and a 6-week whole-stream 15 N enrichment experiment, as part of the Lotic Intersite Nitrogen eXperiment (LINX). Objectives included addressing what made up the diet of these bivalves and whether suspended algae consumed by bivalves were derived from pelagic phytoplankton imported from an upstream lake or attached algae sloughed from instream surfaces. Within the examination of bivalve diets, we considered whether suspension-and/or deposit-feeding modes were employed and whether bivalves selectively assimilated the algal and microbial portions of bulk material they ingested. All 12 unionid species reached a level of 15 N enrichment greater than the bulk suspended organic matter. Sphaerium striatinum (Sphaeriidae) were enriched to levels greater than all presumed food sources. Suspended algae were derived both from sloughed epilithon and pelagic phytoplankton originating from lentic waters upstream. A mixing model suggested that unionids were consuming 80% deposited and 20% suspended material. Alternatively, these bivalves were preferentially assimilating the highly enriched living component of suspended and/or benthic organic matter rather than assimilating the bulk material. These results advance our understanding of freshwater bivalve-feeding ecology, which is necessary if conservation efforts of these increasingly threatened organisms are to succeed.

Differential δ13C and δ15N signatures among scallop tissues: implications for ecology and physiology

“…In consequence, the organism tends to be isotopically enriched in 13 C or 15 N (to respect the equilibrium). But classical enrichment factors have been calculated mainly for terrestrial organisms (De Niro &Epstein, 1978, 81) and a large range is found in the literature (Mingawa and Wada, 1984;Raikow and Hamilton, 2001). We therefore believe that there is a need to validate for each species the isotopic fractionation between diet and tissues.…”

“…First, as already mentioned, a selective assimilation of one component of the SPOM with high isotopic value could occur. Indeed, Raikow and Hamilton (2001) postulated that differential, rather than uniform, assimilation of food resource components within the SPOM could occur. A selective assimilation of nutrients from diet could also be an explanation and further work on stable isotopic composition of the different biochemical fractions of the SPOM would be of interest.…”

“…So, using classical tissue enrichment factors and average stable isotope values of muscle (δ 13 C = -15.2‰; δ 15 N = 9.4‰) and whole body (δ 13 C and δ 15 N calculated whole body = -16.6 and 8.5‰ respectively), one should expect the isotopic ratios of the filter-feeder's diet to be about -16.2 (muscle) or -17.6‰ A classical mixing model (see Raikow and Hamilton, 2001) could predict that Pecten maximus consume a mixed diet with a minority of phytoplankton, or conversely, that it consume mainly phytoplankton together with a small amount of another food of very enriched δ 13 C value. As in mussels, the P. maximus diet might include dissolved organic carbon (Roditi et al, 2000), deposit material (Raikow and Hamilton, 2001), benthic algae, microheterotrophs and bacteria (Kang et al, 1999;Kreeger and Newell, 2001;Nichols and Garling, 2000). As we did not have stable isotope values of these potential food sources in the Bay of Brest, (but only overall POM) we cannot evaluate the scallop's diet in detail.…”

“…Temporal changes in stable isotopes values of the three tissues showed that seasonal variations were most pronounced in the digestive gland. Indeed, this organ has a rapid turn-over and reflects short-term assimilation (Raikow and Hamilton, 2001). We agree with Raikow and Hamilton (2001), that it would be appropriate to use isotope ratios of this organ to assess true seasonal diets of individuals, since the muscle values reflect more longterm integration of nutrient sources.…”

“…Isotopic variations among organs may reflect their different metabolic rates as well as their turn-over rates. For marine organisms, the recent stable isotope enrichment study of Raikow and Hamilton (2001) demonstrated that δ 15 N values of three tissues of unionid mussels reflected short-term and long-term assimilation of nutrients. Differential isotope fractionation occurs among different tissues and establishing a relationship between an organism and its diet is not straightforward (Owens, 1987).…”