ABSTRACT

Small-scale disturbance of seagrass meadows by burrowing shrimps was assessed by mapping and quantifying apparent disturbance marks (burrow openings, sand patches, sand mounds, shafts) and obtaining vertical profiles of sediment properties (grain size composition, organic matter and nutrient contents). The densities and sizes of sediment gaps and burrow openings were determined within sampling quadrats in various meadows and were correlated with ambient bed and site characteristics. Effects on sediment properties were determined in a comparison of disturbed and undisturbed areas in two beds with contrasting organic matter sources. Burrow openings and sediment gaps were common in all the meadows and were associated with either the snapping shrimp Alpheus macellarius Chace, 1988 (Alpheidae, Caridea), or species of Thalassinidea. Alpheid shrimp disturbance was more frequent than that of thalassinidean shrimps. The distribution of sand mounds and associated shafts of thalassinidean shrimps were random, whereas the distribution of sand patches and burrow openings of alpheid shrimps on the beds appeared regular and clumped, respectively. The densities and sizes of sediment gaps and openings were higher and larger in wave-protected than in exposed beds (p < 0.05). Shrimp disturbance was more prevalent in clear-water than in turbid seagrass areas of the Bolinao-Anda region. The distribution of thalassinidean shrimps, however, was wider than that of A. macellarius, which was observed to be

limited to vegetated fine-sand substrates. Overall, alpheid shrimps reworked sand patch areas of 14 ± 2% of the meadows while thalassinidean shrimps produced sand mounds that covered 4 ± 1%.