Comparison of different tracers and methods used to quantify bioturbation during a spring bloom: 234-thorium, luminophores and chlorophyll a

Rates and types of particle bioturbation were measured before and during the 1993 phytoplankton spring bloom in Long Island Sound (U.S.A.). Bioturbation rates were determined at intervals of ~2 weeks using three independent tracers (²³⁴Th, chlorophyll a and luminophores) that have different boundary conditions and reaction properties. ²³⁴Th (t( 1/4 ) = 24 days) is supplied continuously from overlying water. The delivery of chlorophyll a (Chl a), a non-conservative tracer of fresh organic matter having a known degradation rate, tracks overlying water productivity and is characterized by non-steady-state pulses. Luminophores, fluorescent particulate tracers, were used in a series of manipulative pulse input experiments under both in situ and laboratory conditions. Sediment temperature varied from 0-4 to 15.8 °C during the measurement period. Bioturbation was quantified by applying either steady- or non-steady-state biodiffusive-bioadvective models that take into account both diffusive transport (D(b)) and vertical advective transport (V(b)) by 'conveyor-belt' feeders. Pulsed inputs of Chl a and luminophores demonstrated rapid biogenic burial of surface sediment that was not resolved by ²³⁴Th. There was discernible small-scale spatial heterogeneity in mixing as indicated by large variations in bioturbation coefficients estimated in duplicate (field) or triplicate (lab) luminophore cores. During the sampling period, both types of biological sediment transport, biodiffusive mixing and bioadvection, were low (D(b) ranged from 0.001-0.04 cm² day^-1 and V(b)=0.00-0.05 cm day^-1). Biodiffusive mixing estimated from in situ tracer profiles increased (≤2-3 x) during the pulsed organic matter input and temperature rise that characterized the bloom and post-bloom period. Laboratory measurements of luminophores using three replicate cores from each sampling time also indicated a post-bloom increase in biodiffusion with a possible transient peak enhancement (≤4 x) about 40 days after the bloom. There was an apparent short-term relationship between fresh organic flux to the sea-floor and elevated biodiffusion but the most consistent post-bloom relationship is between mixing rate and temperature (apparent activation energies ~45-60 kJ mol^-1). Both in situ and laboratory measurements also demonstrated that bioadvection coefficients increased (~10-50 x) with Chl a input and rising temperature, and correlated directly with specific changes in benthic community composition. Diffusive mixing coefficients calculated from the different tracers are of the same order of magnitude. Use of ²³⁴Th resulted in the lowest estimates of overall particle transport (combined diffusive and advective). Multiple tracers and replication of mixing coefficient measurements are required for accurate quantification and resolution of bioturbation.