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

The small-scale distribution of several structural (bacterial abundance, phytopigment, total and soluble protein, and carbohydrate content) and functional ecological variables (enzymatic activities, frequency of dividing cells) was investigated in coastal sediments during a spring bloom. For bacterial abundance, enzymatic activity, and organic matter determinations, samples were collected at 5-m depth from a sediment surface delimited by a 42 × 42-cm frame, divided into 49 squares. In order to test the influence of the bottom microtopography on the investigated variables, the size of this frame was defined to cover the distance between two subsequent ripples. As indicated by Fisher's index, benthic bacteria, enzymatic activities, proteins, carbohydrates, and their soluble fractions exhibited an aggregate distribution. Sampling size (i.e., sample unit of 36 cm2) was appropriate for all variables, except for chlorophyll a and frequency of dividing cells that displayed a contagious distribution. To estimate the reliability of the current sampling strategy, we compared the mean values from three randomly selected 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 correlate with their potential food sources (e.g., phytopigments, proteins, and carbohydrates), or with enzymatic activities, suggesting the presence of possible time lags between organic inputs and microbial response. Chlorophyll a during the spring bloom displayed much higher concentrations than phaeopigments and correlated with carbohydrates. Chlorophyll a distribution was autocorrelated and displayed a large patch size (1,134 cm2). Phaeopigments and proteins accumulated in the central depression of the ripple-mark structure, apparently depending upon a passive accumulation due to the reduced current action. In contrast, microphy- tobenthic biomass and bacterial numbers were highest in the two opposite ripples, suggesting that different driving forces operate selectively on the living components.