Crayfish and fish as bioturbators of streambed sediments: Assessing joint effects of species with different mechanistic abilities

Many studies illustrate that bioturbating animal species individually affect aquatic sediments through diverse mechanistic abilities, whereas assessments of joint effects of such species on sediments are relatively rare. Such joint effects have implications for real systems, in which different bioturbators coexist, but are difficult to predict for two reasons. First, they can be additive (being the sum of the individual effects of each species) or they can be positive or negative interactive (being greater or smaller than the sum of the individual effects). Second, if interactive, they can depend on biotic interactions that affect the bioturbating activities of the species and/or they can depend on physical interactions among bioturbator-induced sediment modifications. Using experimental streams, we assessed such joint effects on gravel–sand sediments for flow and sediment conditions preferred by barbel (Barbus barbus) but also used by gudgeon (Gobio gobio) and, in a second experiment, for flow and sediment conditions preferred by both male crayfish (Orconectes limosus) and gudgeon. These species have different mechanistic abilities to affect gravel and/or sand in stream beds. In each experiment, we measured (i) the transport of gravel and sand at baseflow (during 12 experimental days); (ii) four sediment surface characteristics (after 12 d); and (iii) the critical shear stress (τc) causing incipient gravel and sand motion during experimental floods (after 12 d). Gudgeon contributed differently to the joint effects in the two experiments, which related to its individual weight, prevailing baseflow shear stress, sediment particle weight, and sediment mixture (availability of surface sand). Overall, the species pairs had predominantly negative interactive joint effects on the sediment variables assessed by us. Both a literature survey and observations during the experiments provided no evidence for direct biotic interactions between barbel and gudgeon or crayfish and gudgeon, so one would reasonably associate their negative interactive effects on the sediments with physical interactions among bioturbator-induced sediment surface modifications. Individually, each species reduced the percentage of sand in the surface layer and the surface algal cover to relatively low values so that the species pairs could not accomplish much greater joint effects on these variables, explaining their negative interactive effects on them. As algal cover particularly affected the τc for gravel and sand, the negative interactive effects of the animals on this surface variable chained toward the τc for the sediments, on which the species pairs also had negative interactive effects. Such chained negative interactive effects on sediment variables are seemingly a general pattern of joint bioturbator effects on aquatic sediments, i.e., the many so far described single-species effects should be smaller than their sum if the species coexists in nature.