International Journal of Marine Science 2015, Vol.5, No.13, 1-11
3
1.2
Experimental design
An experimental method similar to the one described
by Williams and Hynes (1976) was used to determine
the stages of colonization of artificial substrate. Five
rectangular cages of 2x4x5 m in dimension
constructed with galvanized iron to avoid rusting were
used for this study. A polyethylene material of mesh
size of 12 mm was layed to cover the bottom of each
cage, while the four sides and top had openings of
50x50 mm in dimension to allow for the flow of water
and materials. Cages were held firmly in place by rope
tied to iron rod inserted into the sediment.
Five different formulated substrates which served as
content for ten cages (including five replicates) (Table 1)
were used for this study. Cage 1 contained oven dried
sediment from the Lagos Lagoon, this served as the
control. The sediment was oven dried to ensure that
no living organism was left. Cage 2 contained 2 kg of
dried mangrove plant,
Rhizophora racemosa
,
material (dried stems and leaves) churned into 13 kg
of dried mud, while cage 3 consist of 2 kg of dried
plant material churned into 13 kg of oven dried sand.
Cages 4 and 5 comprised 8 kg of dried plant churned
into 7 kg of oven dried sand, and 8 kg of dried plant
material churned into 7 kg of oven dried mud.
The
plant
R. racemosa
, was chosen for this experiment
because of its abundance along the shoreline and the
likelyhood that it may constitute a major source of
detrital input in the lagoon.
Table 1 Location, summary of content and some physical attributes of experimental cages
Cage
Latitude
Longitude
Summary of Cage content
Approximate
distance from the
shoreline (m)
Approximat
e depth (cm)
Average H
2
O
temperature
1 (Control) 6
o
51.84”N 003˚49.02”E Approximately 96.5 % sand and 3.5 %
mud
4.58
30.2
30
2
6
o
52.45’N 003
o
9.99E
2 kg of plant material and 13 kg of mud 4.55
28
30.5
3
6
o
35’02”N 003
o
27’23 E 2 kg of plant material and 13 kg of sand 4.10
23.5
31
4
6
o
035’91”N 003
o
28’79”E. 8 kg of plant material and 7 kg of sand 2.95
32
30.2
5
6
o
36’18”N 003
o
29’01”E 8 kg of plant material and 7 kg of mud 3.85
29
30
Collection of samples
Colonizing species were collected by scooping the
upper part of substrate in each cage with a small hand
trowel. Two scoops were taken from each cage during
a particular sampling exercise and the collected
materials washed through a 0.5 mm mesh sieve. The
residue in the sieve for each cage was preserved in
10 % formalin solution and kept in labelled plastic
containers for further laboratory analyses.
Samples of substrate in each of the cages were
collected with the hand trowel for analyses of nutirient
(PO
4
and NO
3
2-
) content and total organic content
(TOC) by removing the topmost 1 – 5 cm. The
samples were kept in properly labeled polythene bags.
Laboratory analysis
In the laboratory, macroinvertebrate samples were
washed to remove the fixative and sorted under a
dissecting
microscope.
Specimens
of
macroinvertebrates were identified to species level
using the identification keys of Edmunds (1978), and
Yankson and Kendal (2001). Sediment analysis
followed the methods outlined in APHA (1998).
Statistical analysis
One-Way analysis of variance (ANOVA) was used to
compare the variations in macroinvertebrate variables
in the experimental cages. When significant variations
are detected, a
post hoc test
using Tukey’s Honestly
Significantly Different (HSD) was performed to
determine the locations of significant differences. The
relationships between biotic and environmental
parameters were determined using Spearman’s rank
correlations. All statistical analyses were performed
with SPSS 10 and Excel 2003; 2007 for Windows.
2 Results
2.1 Physico-chemical characteristics of experimental
cages
Variations in nutrients and total organic content in the
experimental cages are shown in Figures 1 to 3.
Nitrate concentration was highest in cages 4 and 5
where mean nitrate values of 3 mg/L was obtained. A
