Jan Schrijvers

Macrobenthic infauna of mangroves and surrounding beaches at Gazi Bay, Kenya

Twelve sites around Gazi Bay, Kenya, were examined for macrobenthic infauna. Stations differed in human disturbance, vegetation, and sediment type (sandflat, beach, denuded and virgin mangrove sites, with Sonneratia, Rhizophora, Avicennia, Ceriops or Bruguiera vegetation).Sixteen higher taxa were counted; Isopoda, Amphipoda, Polychaeta, Cumacea and Tanaidacea were determined to family level (sometimes to genus or species). Total densities of infauna ranged from 265 to 6025 ind m−2.Gazi mangals had higher macrobenthos densities than other mangrove sediments described in literature and than the Gazi sandflats. The densities found in Gazi sandflats were comparable to similar habitats elsewhere.Virgin mangrove sediments were rich in mud and organic material, and were characterized by high densities of macro-Oligochaeta and Mollusca. Sandflats, beaches, exploited (denuded) and less dense mangroves or mangroves higher in the tidal zone (Bruguiera) were much more sandy and had a high abundance of Polychaeta and Nemertini.Structural patterns in the macrobenthos were only vaguely associated with vegetation type. Human impact (cutting) has resulted in a drastic decrease in densities of macro-infauna, possibly related to a reduction of organic material in the sediment.

The infaunal macrobenthos under East African Ceriops tagal mangroves impacted by epibenthos

The interactions between macrobenthic in- and epifauna of a mid-intertidal Ceriops tagal (Perr.) Rob. stand were studied in order to test the relative importance of epibenthic predation pressure on and resource competition with the macro-endobenthos. The study is the fourth in a series of papers based on exclusion experiments in two different parts of the mangrove system at Gazi Bay in southern Kenya. The permanent and visiting epibenthos were excluded using cages. Densities of the macrobenthic taxa Oligochaeta (families Tubificidae and Enchytraeidae), Amphipoda (genera Grandidierella and Ampelisca), Insecta larvae (family Dolichopodidae), Polychaeta (families Nereidae and Terebellidae), macro-Nematoda (family Oncholaimidae), and Gastropoda, and a broad range of environmental factors were followed over five months of caging. A significant increase of the dominant tubificid population and the polychaete Namalycastis spec. in the cage-covered sediment indicated a positive exclusion effect. The polychaetes seemed to be impacted by epibenthic predation. Resource competition for muddy detritus, however, was evidenced to be the structuring force for the numerically dominant oligochaetes. The bulk of the studied macrobenthic infauna is therefore proposed to be a trophic dead end and to have only a minor interactive position in the mangrove foodweb.

Interactions between epibenthos and meiobenthos in a high intertidal Avicennia marina mangrove forest

Many studies in the muddy intertidal zone of temperate regions haveindicated meiofaunal communities to be mainly affected by epibenthicpredation and disturbance rather than competition. Few studies, however,have dealt with mangrove sediments of tropical areas. In addition to aparallel study in a Ceriops tagal (Perr.) Rob. zone, a manipulativeexclusion technique was used to trace the dominant biological interactionsstructuring the meiobenthos of an East African Avicennia marina (Forsk.)Vierh. mangrove forest.The densities of the major meiobenthic taxa and nematode genera and abroad range of environmental factors were monitored over a depth profile forone year of caging. Cages (1 m2) excluded all epibenthos (> 2 mm)for one year and were procedurally controlled. Procedural and exclusioneffects were traced, using a factorial and mixed ANOVA design. Significant exclusion effects were indicated for oligochaetes and for oneof the dominant epistratal feeding nematode genera. They are discussed interms of epibenthic composition and density, feeding behaviour, foodresources, and the abiotic environment. The conclusion is that the observed meiobenthos (especially oligochaetesand nematodes) is influenced mainly by exploitative or resource competitionwith the epibenthos. The common food source was indicated to be muddydetritus and microalgae. Consequently, the role of the meiobenthos is mainlysituated in an isolated, detrital food web with only minor energy fluxes tothe epibenthos.