Trace metals (Zn, Pb, Cu, Cd) speciation of surface sediments from Makupa
creek, coastal Kenya were determined by sequential extraction procedure. The
procedure was used to extract the trace metals in sediments geochemical phases
(exchangeable, carbonates, Fe-Mn oxides, organic matter/sulphide, and residual).
Trace metals analysis was done using ICP/MS. The trace metal speciation results
indicated that Pb, Zn, and Cd were mainly associated with the exchangeable, carbonates
and Fe-Mn oxides in most of the sampling sites. The highest concentration of
the trace metals were associated with Fe-Mn oxides with Zn concentration at
362.5 μg/g, Pb, 31.5 μg/g. Copper was mostly associated with the organic matter/sulphide
and carbonate at concentration of 117.5 μg/g and 69.9 μg/g respectively.
Generally, trace metals in sediment from Makupa creek were mainly associated
with the bioavailable fractions (BAF) and their ranges were: Pb (60%-98%), Zn
(90%-99%), Cu (70%-91%). It was found therefore, that there was trace
metals enrichment in sediments from Makupa from anthropogenic sources and
bioavailable to biota.
Seeds are a
source of organic (carbon, C) and mineral (nitrogen, N and phosphorus, P)
nutrients for the growing seedling. There is much information on
seed mass and N and P contents, and the relationship between these and seedling
mass. Within the world’s temperate regions, these collectively show that N and
P concentrations remain constant or rise with increase in seed mass and that
seeds are larger and more nutrient-enriched in poorer soils. Seed N and P were
more important than seed C in accounting for seedling mass in 85% of studies we
assessed. In nutrient- and water-limited environments that are not
light-limited, large seeds routinely provision the seedling with N and P that
enhance C-fixation and thus general growth in the first wet season.
This system is so efficient that growth response to soil nutrients may be
negligible in first-year seedlings arising from seeds > 15 mg mass, N
content > 5 mg and P content > 1.6 mg. The elongating
taproot system absorbs nutrients and maintains water uptake as soil water
retreats, enhancing the chances of survival in the first dry season. We outline
an interpretative scenario for the special role of large seeds (>15 mg) in nutrient-
and water-limited environments that recognizes the critical role of N and P for
photosynthesis in ensuring sufficient C-supply to the rapidly descending roots
for effective drought-avoidance by the young plant.