We measured Vibrio spp. distribution and community profile in the tropical estuary of Port Klang and coastal water of Port Dickson, Malaysia. Vibrio spp. abundance ranged from 15 to 2395 colony forming units mL−1, and was driven by salinity and chlorophyll a (Chl a) concentration. However, the effect of salinity was pronounced only when salinity was <20 ppt. A total of 27 Vibrio spp. were identified, and theVibrio spp. community at Port Dickson was more diverse (H′ = 1.94 ± 0.21). However species composition between Port Dickson and Port Klang were similar. Two frequently occurring Vibrio spp. were V. owensii and V. rotiferianus, which exhibited relatively higher growth rates (ANCOVA: F > 4.338, P < 0.05). Co-culture experiments between fast- and slow-growing Vibrio spp. revealed that fast-growing Vibrio spp. (r-strategists) were overwhelmed by slower-growing Vibrio spp. (K-strategists) when nutrient conditions were set towards oligotrophy. In response to resource availability, the intrinsic growth strategy of each Vibrio spp. determined its occurrence and the development of Vibrio spp. community composition.

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Phytoplankton growth (µ) and grazing loss (g) rates were measured monthly by the Landry-Hassett dilution method over a two-year period at both estuarine (Klang) and study, grazing loss was coupled to phytoplankton growth and the ratio of g : μ or grazing pressure which estimates the proportion of primary production grazed was 50% at Klang and lower than at Port Dickson (68%) (t = 2.213, df = 36, p < 0.05). We found that the higher growth rates in an eutrophic system i.e. Klang were not matched by higher grazing loss, and this may have implications for the biogeochemical cycling in coastal waters.

Diatom abundance, biovolume and diversity were measured over a 2-year period along the Straits of Malacca at two stations with upper (Klang) and lower (Port Dickson) states of eutrophication. Diatom abundance, which ranged from 0.2 × 10 to 21.7 × 10 cells L at Klang and 0.9 × 10- 41.3 × 10 cells L at Port Dickson, was influenced partly by nutrient concentrations. At Klang, the diatoms were generally smaller and less diverse (H' = 0.77 ± 0.48) and predominated by Skeletonema spp. (60 ± 32% of total diatom biomass). In contrast, diatoms were larger and more diverse (H' = 1.40 ± 0.67) at Port Dickson. Chaetoceros spp. were the most abundant diatoms at Port Dickson but attributed only 48 ± 30% of total diatom biomass. Comparison of both Klang and Port Dickson showed that their diatom community structure differed and that eutrophication reduced diatom diversity at Klang. We also observed how Si(OH) affected the abundance of Skeletonema spp. which in turn influenced the temporal variation of diatom community at Klang. Our results highlighted how eutrophication affects diatom diversity and community structure.

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We sampled extensively (29 stations) at the Klang estuarine system over a 3-day scientific expedition. We measured physical and chemical variables (temperature, salinity, dissolved oxygen, total suspended solids, dissolved inorganic nutrients) and related them to the spatial distribution of phototrophic picoplankton (Ppico). Multivariate analysis of variance of the physicochemical variables showed the heterogeneity of the Klang estuarine system where the stations at each transect were significantly different (Rao's F₁₈, ₃₆ = 8.401, p < 0.001). Correlation analyses also showed that variables related to Ppico abundance and growth were mutually exclusive. Distribution of Ppico was best explained by the physical mixing between freshwater and seawater whereas Ppico growth was correlated with temperature.

Context We studied the virus component in the microbial food-web in the tropical coastal Sunda Shelf waters. Aims We investigated viral processes, including the lysogenic to lytic shift dynamics with seawater warming and nutrient enrichment. Methods We sampled nearshore and offshore waters of Peninsular Malaysia to provide a natural trophic gradient for correlational analyses. We also conducted laboratory experiments to investigate the changes in virus production rates and lysogeny, when temperature (range: 25–37°C) and nutrient concentrations (range: 0.1–1.0× marine-broth concentration) were manipulated. Key results Observations showed site-dependent differences in dissolved inorganic nutrient concentration, Chl-a (0.21–3.54 μg L−1) and bacterial abundance (4.09 × 105−9.41 × 105 cells mL−1), but not in viral abundance (measured as virus-like particles or VLP) (1.04 × 106−2.39 × 106 VLP mL−1) and virus production rates (0.59 × 105−4.55 × 105 VLP mL−1 h−1). From laboratory experiments, both warming and nutrient enrichment increased virus production (R2 > 0.651), decreased lysogeny (R2 > 0.743), and resulted in an increase of the viral lysis:bacterial production ratio, and a stronger viral control on bacterial production. Conclusions Although both seawater warming and nutrient enrichment increased virus production and reduced lysogeny, nutrient enrichment was a more important factor. Implications The increasing nutrient concentrations in these waters will increase virus production and virus top-down control of bacteria, and drive the ecosystem towards heterotrophy.