Inter Tidal Sand and Mud Flats
The bolongs are tidal for their entire dry season length. Their combined tidal outflow of these bolongs meeting the northerly currents arising from 29 the river Gambia have resulted in a sand bar formation of Buniada Point. The spit is covered on high tides and resultantly has no associated vegetation, though it is a regular roosting site for a variety of sterns gulls, waders and herons.
Along the Masarinko and Niji Bolongs, Numerous mud banks become exposed during low water. No vegetation is associated with these mud banks possibly due to the tidal surge within the bolons. Backing the mangrove fringe of the bolons extensive areas of salt pan (bare tannes) occur where hyper-saline conditions limit the growth of plants.
Colonization by halophytes is generally limited to the peripheries of these pans and consists mainly of Sesuvium portulacatrum, Philoxerus vermicularis, Paspalum vaginatum, and Sorobolus spp.
Inter Tidal Marshes
Halophytic vegetation associated with salt pans has been referred to above and the same complex of species is also associated with inter tidal marshes and seasonally flooded areas. On the island of Jinack the low lying nature of the island (essentially a vegetation spit ) subjects a large portion of the island to seasonal flooding through rainfall.
The salinity of these areas steadily rises due to residual salts from evaporation during the end of the rains and the dominant vegetation is essentially halophytic in nature. Rainfall swamps occur on the eastern side of the island and are utilized for rice cultivation. Part of the seasonally flooded areas is also subject to periodic flooding during spring tides.
Salt marshes are generally fringed by Tamarix senegalensis with occasional Avicennia africana. Adansonia digitata occurs on slightly elevated land fringing the marshes.
Inter Tidal Forests
Mangrove forest dominates the bolongs fringes within the Niumi National Park. The total area of mangrove within the park is approximately 800ha. Six woody species are found within the mangrove belt, namely. Rhizophora harissionii Rhizophora racemosa Laguncularia racemosa Avicennia nitida Conocarpus erectus Rhizophora mangle
All 6 species occur within the Niumi mangroves though the distribution of Rhizophora species has not been investigated in detail. Sukardjo (1995) has outlines four mangrove communities types found in The Gambia based on lugo and Sndaker’s (1974) criteria.
The communities identified are fringe forest, riverine forest, basin forest and scrub or dwarf forest. According to these criteria however it appears that a fifth mangrove community occurs within the Tanbi Wetland complex overwash forest. The fringe mangrove forests are found along waterways where the shoreline elevation is slightly higher than the mean high tide level and the salinity remains fairly constant through out the year.
This forest is composed of monotypic stands of Rhizophora mangle in the outer estuary of the River Gambia and in the Upper stretches of the bolongs within Niumi National Park. All three Rhizophora species are found in this zone and tree height can reach more than 10m. Basin mangrove forests occur in areas subject to tidal inundation during spring tides only and with correspondingly high soil salinity levels.
This forest type is dominated by Avicenma nitida and tree height may get up to 20m. Scrub or dwarf forests are to be found in areas with limited tidal inundation and high salinity levels, often backing the fringe forest. Avicennia predominates but may be accompanied by Rhizohora and Laguncularia. Fringe Mangrove forest is predominant within Niumi and is found along the Masarinko and Niji Bolongs.
The north – east tip of Jinack Island and the Mbankama spit have extensive stands of this forest type which is backed by scrub forest and bare tannes. Stands of riverine mangrove forests are found in the mid and upper tidal reaches of the Masarinko Bolon, reaching heights up to 12m though generally less than 10m.
On the spits opposite and to the south of Bakindik Koto, this forest type occurs on a peat deposit which sporadically (occurs) in the Gambia as thin beds within the fluvial marine sequence (White and Russell, 1988).
Here also, the forest grades to scrub mangrove in the inland reaches. The northern aspect of the park is demarcated by a labyrinth of Rhizophora mangrove creeks and channels that are presently not well researched in seasonal terms.
At low tide in January extensive mud flats are exposed harbouring a diverse range of feeding shorebirds that reflect the avifaunal content of the Bund Road area (Tanbi Wetland Complex). African Fish Eagle (Haliaetus vocifer) and Goliath Heron (Ardea goliath) are commonly seen but not numerous. Ble-cheeked Bee Eater (M. persicus) is 32 particularly abundant also in January.
The creek systems are used by a variety of kingfisher species both resident and regionally migrant