Small-scale distribution of bacteria, enzymatic activities, and organic matter in coastal sediments

The small-scale distribution of several structural (bacterial abundance, ph ytopigment, total and soluble protein, and carbohydrate content) and functi onal ecological variables (enzymatic activities, frequency of dividing cell s) was investigated in coastal sediments during a spring bloom. For bacteri al abundance, enzymatic activity, and organic matter determinations, sample s were collected at 5-m depth from a sediment surface delimited by a 42 x 4 2-cm frame, divided into 49 squares. In order to test the influence of the bottom microtopography on the investigated variables, the size of this fram e was defined to cover the distance between two subsequent ripples. As indi cated by Fisher's index, benthic bacteria, enzymatic activities, proteins, carbohydrates, and their soluble fractions exhibited an aggregate distribut ion. Sampling size (i.e., sample unit of 36 cm(2)) was appropriate for all variables, except for chlorophyll a and frequency of dividing cells that di splayed a contagious distribution. To estimate the reliability of the curre nt sampling strategy, we compared the mean values from three randomly selec ted sample units with the average value of the entire sediment surface (i.e ., 49 samples). For all variables reported in this study, the use of three replicates was representative of the mean values of the sampled area with a confidence limit within +/- 20%. Bacterial population sizes did not correl ate with their potential food sources (e.g., phytopigments, proteins, and c arbohydrates), or with enzymatic activities, suggesting the presence of pos sible time lags between organic inputs and microbial response. Chlorophyll a during the spring bloom displayed much higher concentrations than phaeopi gments and correlated with carbohydrates. Chlorophyll a distribution was au tocorrelated and displayed a large patch size (1,134 cm(2)). Phaeopigments and proteins accumulated in the central depression of the ripple-mark struc ture, apparently depending upon a passive accumulation due to the reduced c urrent action. In contrast, microphy-tobenthic biomass and bacterial number s were highest in the two opposite ripples, suggesting that different drivi ng forces operate selectively on the living components.