Phytoplankton
is the primary producer of organic food supply in the marine food chain
and therefore an important basic link in the production and sustenance of
life at sea, including fish. Studies on species composition, abundance and
distribution of phytoplankton identified 133 species of diatoms [55 genera],
107 species of dinoflagellates [31 genera] and
two species of bluegreen algae [2 genera], of which
17 diatoms and one blue-green alga were predominant. The genera Chaetoceros,
Rhizosolenia, Coscinodiscus, Bacteriastrum and Ceratium had the
widest range of species. The most dominant diatom species were Chaetoceros
lorenzianus, C. costatum, Thalassionema frauenfeldii, Skeletonema costatum,
Pleurosigma elongatum, Bacteriastrum comosum, Bacillaria paxillifera, Coscinodiscus
jonesianus and Rhizosolenia calcar-avis. Dinoflagellates were
usually more common in the offshore waters and were predominated by Ceratium
fusus, C. pentagonum, C. arietinum, Protoperidinium sp., Protoceratium
sp., Ceratocorys sp. and Alexandrium sp.
Diversity index ranged from 1.95 to 3.4 in the offshore areas, with increasingly high values towards the coast, and followed a very similar trend during both the seasons. The evenness index was directly proportional to the diversity index. Percentage abundance of diatoms during both the cruises was high in the Upper Gulf, ranging from 63 - 91%, which decreased drastically to 6 - 22% in the offshore areas. Studies on distribution patterns through station cluster analysis clearly show patchiness of the standing crop, where phytoplankton aggregates at localized foci of abundance in usually near-shore semi-enclosed areas with more stable conditions, except for Trichodesmium, accumulation patterns of which, as a predominantly surface species, are solely determined by prevailing wind patterns, mainly wind direction and speed.
Phytoplankton standing
crop was particularly abundant in the Upper Gulf during the pre-NE monsoon
cruise, withthe highest cell counts near the west coast [214 to 33,520 cells/l]
[Fig.11a]. Diatoms were the predominant
group, closely following the distribution pattern of total phytoplankton
[Fig.12a]. Thalassionema frauenfeldii,
was the most dominant diatom near the coastal areas, while Chaetoceros
lorenzianus attained maximum cell counts in the central areas of the
Upper Gulf and the southern tip of Peninsular Malaysia. Species of Bacteriastrum,
Rhizosolenia, Coscinodiscus, Hemiaulus, Ceratium and Protoperidinium
were also common. Dinoflagellate counts were generally low except in the
Upper Gulf and offshore regions of the Gulf, where localized blooms with
a total abundance ranging from 51% to 76% were recorded. Bluegreen algae
attained peaks along the eastern parts of the Upper Gulf, in the Central
Gulf and along the East Coast of Peninsular Malaysia. Trichodesmium erythraeum
was dominant covering large offshore areas, attaining peak density values
of 2.24 x 106 filaments/m3 [93.4% of total density] [Fig.13].
Phytoplankton densities were generally higher after the NE monsoon, with the richest standing crop along the East Coast of Peninsular Malaysia [178 – 14,223 cells/l] [Fig.11b]. Once again, diatoms, as the predominant group, closely followed the distribution pattern of total phytoplankton [Fig.12b]. Chaetoceros compressus, C. affinis, C. didymus and Pleurosigma sp. were more common in the coastal areas of the Gulf, while Proboscia alata, Pseudosolenia calcar-avis, Bacteriastrum comosum, Chaetoceros peruvianus and Cylindrotheca closterium were abundant in the offshore areas. Species of Coscinodiscus, Rhizosolenia, Thalassionema, Hemiaulus, Ceratium and Protoperidinium were also common. Species diversity indices were particularly high in the coastal areas, except within the blooms, such as that of the neritic diatom, Skeletonema costatum, which reached a density of 113,336 cells/l and a relative abundance of 90.9% towards the southern tip of Peninsular Malaysia. Concentrations of the diatom, Pleurosigma sp. had the highest relative abundance of up to 91.4% [14,223 cells/l] at some stations in the Upper Gulf. Dinoflagellate densities were higher along the Malaysian coast as compared to that in the Gulf. Blue-green algae distribution, especially Trichodesmium erythraeum, was more patchy, with peaks along coastal areas of Peninsular Malaysia, that sometimes reached bloom conditions, and the lowest concentrations in the Central Gulf [Fig.13].
Although
the species composition was rather similar throughout, phytoplankton standing
crop displayed a clear pattern of patchiness, but was generally concentrated
in the uppermost and lower regions of the survey area, with a wide area
of low concentrations in the Central Gulf [Fig.11],
that could be related to nutrient enrichment and current flow patterns respectively.
Increase in the abundance of phytoplankton standing crop after the NE monsoon
indicates the effects of the monsoon and the
conditions it brings about in stimulating phytoplankton
growth in the area. Diatoms contribute to the bulk of the standing crop,
the distribution patterns of which clearly follow that of the total phytoplankton
standing crop [Fig.12]. This, together
with low concentrations of dinoflagellates, show the strong neritic influence
existing in the area. Large species diversity, especially in the coastal
areas, clearly follow the pattern expected in normal tropical marine coastal
ecosystems. Richness index passed the 2.5 level at some offshore stations
during the pre-NE monsoon season, which moved near-shore after the monsoon.
Species diversity index ranged wider during pre-NE monsoon especially in
the Gulf, which narrowed after the monsoon, mainly because of the tendency
of some species to proliferate to bloom conditions, as a result of which
the evenness indices were also generally higher during this season. Blooms
of only one diatom, Skeletonema costatum were recorded after the
NE monsoon towards the southern tip of the survey area. Bacteriastrum comosum
and Chaetoceros lorenzianus were associated with this bloom. Species
associations of Ceratium, Coscinodiscus and Hemiaulus were also noticed.
Ceratium fusus and Nitzschia closterium also showed a tendency
towards eco-specific associations with blooms of the bluegreen alga, Trichodesmium
erythraeum. On the whole, diatoms contribute to the bulk of primary
production recorded in the area, except at those offshore stations where
blue-green algae or in rare cases some dinoflagellates were predominant.
Toxic dinoflagellate species were quite negligible throughout, which goes
to explain the lack of ‘red tides’ in the study area. There are some indications
that planktonic community organization in the water column might have undergone
subtle changes in the recent past, especially in the dominance pattern of
some species, probably in response to some recent environmental changes
such as increase in pollution levels. However, in-depth analyses are needed
to verify this assumption. Studies have shown that distribution and abundance
of pelagic fish species such as the little tuna [Euthynnus affinis],
chub mackerel [Rastrelliger sp.] and anchovies [Stolephorus
spp.] are related to the density of phytoplankton.
Life
cycles of dinoflagellates generally alternate between a vegetative stage
[motile cell in the phytoplankton]
and a resting stage [non-motile cell or cyst mostly
in the sediments] produced during the sexual phase. They also act as the
seed population to trigger potentially toxic red tides. Cysts of 20 dinoflagellate
species, belonging to Goniolacoid, Tuberculodinioid and Peridinioid groups
were collected from surface sediments in the survey area. Cysts were mostly
found at depths of more than 30 m and in low densities, which may be caused
by the constant disturbance of the surface sediments by frequent trawling
activities in the nearshore areas. Spiniferites spp. [cyst phase
of Gonyaulax spp.] was the most dominant cyst throughout, but was
more common and widely distributed in Malaysian waters. [However, red tides
of Gonyaulax have never been reported from the survey area so far.]
Protoperidinium spp. and Scrippsiella sp. were found only
in the Gulf and Operculodinium centrocarpum and Tuberculodinium
vancampoae were restricted almost entirely to the Malaysian waters.
Cysts of toxic ‘red tide’ species were recorded only in very negligible
numbers. Protoperidiniacean dinoflagellate abundance is closely associated
with high diatom productivity and availability of nutrients.
Although many tropical species of coral fish and some pelagic fish are known to be primarily herbivorous [mainly feeding on seaweeds], previous studies have shown that such predominantly herbivorous [phytoplankton feeding] species are negligible in the Gulf, except perhaps in the vicinity of coral reefs. Only Leiognathus bindus, an omnivorous feeder, was seen to ingest diatoms in some quantities. Therefore, phytoplankton seems to be of significance to the food chain in the area mainly as food for zooplankton, fish larvae and possibly juveniles of fish.