Small-scale spatial and temporal heterogeneity in growth and condition of juvenile fish on sandy beaches

To characterize fine-grained heterogeneity in the functional role of sandy beaches as fish nurseries, we measured RNA concentration, DNA concentration and RNA-predicted growth rate in young-of-the-year European plaice Pleuronectes platessa L. (‘YOY plaice’) at beaches on the western side of the British Isles at small alongshore (100–500m), temporal (days to weeks), depth (0.5–2m below waterline) and inter-individual scales. We compared variations in these nucleic-acid-based metrics of condition and growth with a null metric (total length) considered to be less responsive to short-term changes in environmental conditions. All four metrics varied at small spatial and temporal scales, particularly RNA-based metrics of short-term condition and growth. A decline in condition and growth during summer was the most important source of temporal variation. Condition and growth also increased with depth below waterline. Alongshore variations within a beach were relatively small, but variance among stations on the same beach (500m scale) was similar in magnitude to variance among beaches separated by 100km. High variance in nucleic acid concentrations among samples resulted not only from technical error but also from inter-individual variation. Our application of high-resolution biochemical indices therefore demonstrates that functional roles of sandy beaches can be structured at remarkably small spatial (500m) and temporal (days) scales, within a single beach. •We measure fine-scale variation in YOY plaice growth and condition indices.•Temporal variation was detected at daily and weekly scales.•Spatial variation was detected along the depth gradient.•Alongshore spatial variation at ≤500m was small, but similar to 100km scales.•Our study demonstrates the potential for nursery processes to vary at small scales.