Degradation of mangrove leaf litter in the St Lucia Estuary as influenced by season and exposure

Abstract

Rates of degradation of leaves of Avicennia marina (Forssk.) Vierh. and Bruguiera gymnorrhiza (L.) Lam. were investigated in St Lucia Estuary in the warm and cool seasons at three positions on the shore, viz. where leaves were constantly submerged, where they were exposed only at low spring tides and where they were inundated only at high spring tides. Degradation occurred more rapidly in A. marina than in B. gymnorrhiza and leaves constantly submerged decomposed at a faster rate than those exposed for most of the time. Decomposition rates were consistently lower in the cool season. The concentration of nitrogen in all leaves increased over the decomposition period, although there was a decrease in total nitrogen content with rapid decomposition. While under certain conditions degradation proceeds at a rapid rate, because of the lower additions of nitrogen to the system from decomposing leaves, there do not appear to be significant ecological advantages from this rapid decomposition. There were sharp decreases in the phosphorus and potassium contents of the leaves, presumably largely due to leaching. This work has emphasized the importance of mangrove leaf litter as a source of, and substrate for, nutrient release. Die afbraaktempo van blare van Avicennia marina (Forssk.) Vierh. en Bruguiera gymnorrhiza (L.) Lam. is gedurende die warm en koel seisoene op drie verskillende plekke op die kus van die St Luciariviermond ondersoek, naamlik waar die blare deurlopend ondergedompel is, waar dit slegs gedurende lae springgety ontbloot is en waar dit slegs gedurende hoë springgety bedek word. Afbraak het vinniger plaasgevind by A. marina as in B. gymnorrhiza en blare wat deurlopend ondergedompel is het teen 'n vinniger tempo vergaan as dié wat vir die grootste deel van die tyd ontbloot was. Afbraaktempo's was deurgaans laer gedurende die koelseisoen. Die stikstofkonsentrasie in al die blare het gedurende die afbraakperiode toegeneem alhoewel daar tydens snelle afbraak 'n afname in totale stikstofinhoud was. Alhoewel daar onder sekere toestande afbraak teen 'n versnelde tempo plaasvind, wil dit voorkom asof daar, vanweë die stadiger toevoeging van stikstof tot die sisteem van die verrottende blare, nie 'n merkbare ekologiese voordeel van die vinnige afbraak is nie. Daar was skerp afnames in die fosfaat- en kaliuminhoud van die blare, vermoedelik hoofsaaklik vanweë uitloging. Hierdie werk het die belangrikheid van manglietblaarval as 'n bron van, en substraat vir vrystelling van voedingstowwe beklemtoon.