Effects of food resources on the fatty acid composition, growth and survival of freshwater mussels

Abstract

Increased nutrient and sediment loading in rivers have caused observable changes in algal community composition, and thereby, altered the quality and quantity of food resources available to native freshwater mussels. Our objective was to characterize the relationship between nutrient conditions and mussel food quality and examine the effects on fatty acid composition, growth and survival of juvenile mussels. Juvenile Lampsilis cardium and L. siliquoidea were deployed in cages for 28 d at four riverine and four lacustrine sites in the lower St. Croix River, Minnesota/Wisconsin, USA. Mussel foot tissue and food resources (four seston fractions and surficial sediment) were analyzed for quantitative fatty acid (FA) composition. Green algae were abundant in riverine sites, whereas cyanobacteria were most abundant in the lacustrine sites. Mussel survival was high (95%) for both species. Lampsilis cardium exhibited lower growth relative to L. siliquoidea (p <0.0001), but growth of L. cardium was not significantly different across sites (p = 0.13). In contrast, growth of L. siliquoidea was significantly greater at the most upstream riverine site compared to the lower three lacustrine sites (p = 0.002). In situ growth of Lampsilis siliquoidea was positively related to volatile solids (10 - 32 μm fraction), total phosphorus (<10 and 10 - 32 μm fractions), and select FA in the seston (docosapentaeonic acid, DPA, 22:5n3; 4,7,10,13,16-docosapentaenoic, 22:5n6; arachidonic acid, ARA, 20:4n6; and 24:0 in the <10 and 10 - 32 μm fractions). Our laboratory feeding experiment also indicated high accumulation ratios for 22:5n3, 22:5n6, and 20:4n6 in mussel tissue relative to supplied algal diet. In contrast, growth of L. siliquiodea was negatively related to nearly all FAs in the largest size fraction (i.e., >63 μm) of seston, including the bacterial FAs, and several of the FAs associated with sediments. Reduced mussel growth was observed in L. siliquoidea when the abundance of cyanobacteria exceeded 9% of the total phytoplankton biovolume. Areas dominated by cyanobacteria may not provide sufficient food quality to promote or sustain mussel growth.

Fig 1. Sampling sites (red triangles) within the St. Croix National Scenic Riverway.

Fig 2. Mean growth (difference between the initial and final length) of juvenile Lampsilis cardium and L. siliquoidea deployed for 28 d at four riverine and four lacustrine sites in the St. Croix River.

Unless noted n = 3. Error bars represent ± 1 SE. An * denotes significant differences, Tukey’s HSD, p <0.05.

Fig 3. Canonical scores plot derived from a discriminant analysis of the concentrations of 18 fatty acids in the foot tissues of Lampilis cardium and L. siliquoidea grouped by site in the St. Croix River during a 28 d in situ exposure.

Open and closed symbols are from juveniles deployed at riverine and lacustrine sites, respectively. The centroid for each 95% confidence region is denoted by a +.

Fig 4. Differences in the concentration of selected fatty acids in the foot tissue of Lampsilis cardium and L siliquoidea located at a given site (i.e., four riverine or four lacustrine sites in the St. Croix River during August 2008).

Positive values indicate a greater concentration in L. siliquoidea compared to L cardium. Error bars represent ± 1 SE.

Fig 5. Fatty Acid Accumulation Ratios (FAAR) for Lampsilis cardium and L. siliquoidea held in a laboratory for 28-d and fed a commercial Nannochloropsis diet.

FAAR represents the ratio of the 28-d net tissue accumulation of a specific fatty acid divided by the concentration of that fatty acid in the diet for a given mussel species.

Fig 6. Total Phosphorus (TP, mg-P/L), Total Nitrogen (TN, mg-N/L), Total Suspended Solids (TSS, mg/L), Volatile Suspended Solids (VSS, mg/L), and chlorophyll a (mg/m³) in water samples by size fraction collected from four riverine and four lacustrine sites in the St. Croix River during August 2008.

River mile indicated under site designation.

Fig 7. Nitrate-Nitrite (NO₃-NO₂, mg-N/L), Total Ammonia Nitrogen (TAN, mg-N/L), and Dissolved Inorganic Nitrogen (DIN, mg-N/L) in water samples collected (n = 2 unless noted) from four riverine and four lacustrine sites in the St. Croix River during August 2008.

Error bars represent the range.

Fig 8. Biovolume (mm³/L) of phytoplankton in whole water samples collected from four riverine and four lacustrine sites in the St. Croix River during August 2008.

Fig 9. Correlation loading plot for the Partial Least Squares (PLS) Regression of Lampsilis siliquoidea growth.

The plot is derived from a reduced set of predictor variables having a Variable Important to Projection (VIP) value ≥1.25 in the full model. The sampling sites (i.e., observations, labeled R1 – R4 for riverine and L5 – L8 for lacustrine) are located relative to their X score for each PLS factor. The location of the sites orthogonal to the vector (black line) labeled “Growth” indicate the relative magnitude of the growth response (e.g., L5 and R1 having highest growth with sites L6, L7, L8 exhibiting low or no growth). Variables located nearer the label “Growth” along the diagonal line passing through the origin, are highly correlated with growth relative to those variables located opposite of growth, which are negatively related to growth. The majority of the variation in “Growth” was explained by PLS factor 1 (YR² = 74.5%), with an additional 20.6% of variation explained by PLS factor 2. The combined variation explained by model factors 1 and 2 is 94.5%.

Fig 10. Partial Least Squares (PLS) model coefficients for water column and sediment variables used to predict Lampsilis siliquoidea growth.

The direction (+ or -) and length of the horizontal bars relative to the origin indicate the relationship and strength of these predictors to L. siliquoidea growth based on centered (mean = 0) and scaled (standard deviation = 1) data for the full PLS model. The bar coloration specifies Variable Importance to Projection (VIP), such that red bars indicate higher VIP scores and increased contribution to the overall model for both the predictor and the response variables.

Fig 11. Conceptual framework relating predictor variables to observed patterns in patterns in growth.

Red arrows imply positive relationship, blue arrows indicate a negative relationship.