Resting Stage of Plankton Diversity from Singapore Coastal Water: Implications for Harmful Algae Blooms and Coastal Management.

Resting strategies of planktonic organisms are important for the ecological processes of coastal waters and their impacts should be taken into consideration in management of water bodies used by multiple industries. We combined different approaches to evaluate the importance of resting stages in Singapore coastal waters. We used molecular approaches to improve the knowledge on Singapore biodiversity, we sampled and extracted cysts from sediments to evaluate the density of resting stages in Johor Strait, and we compared systematically information on Singapore planktonic biodiversity to existing published information on resting stages from these reported organisms. This is the first study evaluating the importance of resting stages in Singapore waters. Above 120 species reported in Singapore are known to produce resting stages though no previous work has ever been done to evaluate the importance of these strategies in these waters. The results from the resting stage survey confirmed 0.66 to 5.34 cyst g-1 dry weight sediment were present in the Johor Strait suggesting that cysts may be flushed by tidal currents into and out of the strait regularly. This also suggest that the blooms occurring in Singapore are likely due to secondary growth of Harmful Algae Bloom species in the water rather than from direct germination of cysts from sediment. Finally, we discuss the importance of these resting eggs for three main national industries in Singapore (shipping, marine aquaculture and provision of drinking water through seawater desalination). We argue that this study will serve as a baseline for some of the future management of Singapore waters.

Changes in free amino acid content during naupliar development of the Calanoid copepod Acartia tonsa.

Changes in free amino acids (FAA) were investigated in the potentially important live feed and neritic copepod species Acartia tonsa during naupliar development. Total content of FAA in A. tonsa nauplii was around 17% of dry weight at first development stage, and declined to 6% for later stages. Relative to body-volume and biomass, the FAA content indicated possible volume-dependent changes. However, changes in FAA with osmolytic activity could not account for this decline in FAA content, but suggests that the decline reflected degradation of residual FAAs from the embryonic stage. Glutamic acid revealed the largest change in relative abundance during naupliar development and declined from 29.0% at first nauplius stage to 7.1% at later stages. The high FAA pool in early naupliar stages may be necessary for naupliar development due to an absence of feeding at first development stages. The high FAA content in early nauplii indicates that A. tonsa is a valuable source for nutritional energy for first-feeding fish larvae and should be further exploited for aquaculture purposes. Enhancements to FAA abundances in nauplii through manipulation of maternal diets could be of future interest, as copepod nauplii can contain a substantial pool of FAAs at first development stage.

Improvement in compliance of ships' ballast water discharges during commissioning tests.

The number of ships installing ballast water management systems (BWMS) has risen steeply since the Ballast Water Management Convention entered into force. Since June 2022, biological testing is required during commissioning to verify compliance with the Convention. Data from 676 tests (from 2019 to 2022) show substantial improvement over time: the failure rate decreased from ~20 % to ~6 %. Notably, nearly all failures occurred in the largest size class of organisms (≥50 µm). Interestingly, proxy measurements suggest that high concentrations of living organisms in uptake water did not cause the failures. Also, failures determined using "indicative" analysis (here, adenosine triphosphate, ATP) were typically not confirmed by "detailed" analysis (microscopy), suggesting that ATP limits are over-precautionary. Finally, discharges containing high levels of Total Residual Oxidants (TRO) decreased over time. These data highlight the need for ongoing testing-focusing at least on organisms ≥50 µm-to minimize environmental risks from organisms transported in ships' ballast water.

Design and installation of ballast water sample ports: Current status and implications for assessing compliance with discharge standards.

To verify ships' compliance with ballast water regulations, samples may be collected and tested for viable organisms. This task is completed using a sample probe, which is placed in the ballast discharge pipe through a sample port (a flanged opening). To collect representative samples, the placement of the sample port and the size of the sample probe must be appropriate for the shipboard piping arrangement and ballast water flows. The placement of sample ports was evaluated on 72 ships to assess the current condition of ballast water sampling installations against available guidance. Few ships (15%) had sample ports fully aligned with International Organization for Standardization (ISO) standard 11711-1. While current configurations may present challenges in collecting representative samples, these installations likely occurred before the ISO standard was available. Future installations should be in accordance with the standard to facilitate representative sampling.

Use of standard test organisms for sound validation of UV-based ballast water treatment systems.

To challenge ballast water treatment system (BWTS) efficacy for organisms in the size-class 10-50 µm, intake concentration during tests must reach certain minimum requirements. Often, natural concentrations are too low to meet intake requirements and standard test organisms (STOs) are added. We tested the robustness of Tetraselmis suecica and Odontella sp. to a range of UV-treatments to explore fluences needed to meet the IMO discharge standard (<10 org. ml-1) evaluated using two viability assessment methods. To meet discharge standards, fluences of >1000 mJ cm-2 were required using vital stain whereas 135-500 mJ cm-2 were needed using regrowth assays. Besides, results suggest that T. suecica and Odontella sp. were at least as robust as natural algae towards UV-treatments. We suggest the advantageous use of these species as STOs in test water to support intake water requirements and to obtain more conservative validation of UV-based BWTS to ensure more environmental protective procedures.

Effects of concentration and size of suspended particles on the ingestion, reproduction and mortality rates of the copepod, Acartia tonsa.

Suspended sediments are a common occurrence in the marine environment. They can be generated by natural causes, including waves and currents, or brought about by anthropogenic activities such as reclamation and dredging. High sediment concentrations are known to have negative consequences on copepods; however, the impact of sediment size has largely been overlooked. Here we examine the effects of sediment size and concentration in combination with varying algae concentrations on the ingestion rate, egg production, hatching success and survivorship of the copepod species, Acartia tonsa. High concentration of 'small' sediments at 'low' food availability had the greatest negative impact on all parameters except hatching success. Greater food concentration was able to mitigate some of these effects. High concentrations of 'large' sediments also reduced egg production rates, possibly due to A. tonsa avoiding falling particles. We conclude that it is important to examine the particle size distribution when evaluating the impacts of suspended sediments on copepods.

UV fluences required for compliance with ballast water discharge standards using two approved methods for algal viability assessment.

This study investigates the extra UV fluence needed to meet the International Maritime Organisation's ballast water discharge standards for the 10-50 µm size-class using the approved vital stain (VS) method compared to the Most Probable Number (MPN) method for organism viability assessment. Low- and medium pressure UV collimated beam treatments were applied to natural algae collected in temperate and tropical water environments and analysed using both methods. About 10 times higher UV fluence was required to meet discharge standards when using VS compared to MPN. Implementing a dark-hold period after UV treatments decreased algal viability. Length of dark-hold period to meet discharge standards decreased with increasing UV fluence. No significant differences between temperate and tropical samples were observed. The results showed that UV treated algae assessed using the VS method could meet discharge standards by increasing fluence and/or introducing a dark-hold period.

Non-proportional bioaccumulation of trace metals and metalloids in the planktonic food web of two Singapore coastal marine inlets with contrasting water residence times.

We analyzed the concentrations of trace metals/metalloids (TMs) in the water, sediment and plankton of two semi-enclosed marine coastal inlets located north of Jurong Island and separated by a causeway (SW Singapore; May 2012-April 2013). The west side of the causeway (west station) has residence times of approximately one year, and the east side of the causeway (east station) has residence times of one month. The concentrations of most of the TMs in water and sediment were higher in the west than in the east station. In the water column, most of the TMs were homogeneously distributed or had higher concentrations at the surface. Preliminary evidence suggests that the TMs are primarily derived from aerosol depositions from oil combustion and industry. Analyses of TMs in seston (>0.7µm; mostly phytoplankton) and zooplankton (>100µm) revealed that the seston from the west station had higher concentrations of most TMs; however, the concentrations of TMs in zooplankton were similar at the two stations. Despite the high levels of TMs in water, sediment and seston, the bioaccumulation detected in zooplankton was moderate, suggesting either the presence of effective detoxification mechanisms or/and the inefficient transfer of TMs from primary producers to higher trophic levels as a result of the complexity of marine planktonic food webs. In summary, the TM concentrations in water and seston are not reliable indicators of the bioaccumulation at higher trophic levels of the food web.

Effects of eutrophication on the planktonic food web dynamics of marine coastal ecosystems: The case study of two tropical inlets.

We studied the plankton dynamics of two semi-enclosed marine coastal inlets of the north of Jurong Island separated by a causeway (SW Singapore; May 2012-April 2013). The west side of the causeway (west station) has residence times of ca. one year and is markedly eutrophic. The east side (east station) has residence times of one month and presents lower nutrient concentrations throughout the year. The higher nutrient concentrations at the west station did not translate into significantly higher concentrations of chlorophyll a, with the exception of some peaks at the end of the South West Monsoon. Microzooplankton were more abundant at the west station. The west station exhibited more variable abundances of copepods during the year than did the east station, which showed a more stable pattern and higher diversity. Despite the higher nutrient concentrations at the west station (never limiting phytoplankton growth), the instantaneous phytoplankton growth rates there were generally lower than at the east station. The phytoplankton communities at the west station were top-down controlled, largely by microzooplankton grazing, whereas those of the east station alternated between top-down and bottom-up control, with mesozooplankton being the major grazers. Overall, the trophic transfer efficiency from nutrients to mesozooplankton in the eutrophic west station was less efficient than in the east station, but this was mostly because a poor use of inorganic nutrients by phytoplankton rather than an inefficient trophic transfer of carbon. Some hypotheses explaining this result are discussed.

Effects of temperature on type approval testing of ballast water treatment systems.

To limit the risk associated with invasion of habitats by exogenous species, the International Convention for the Control and Management of the Ships' Ballast Water and Sediments was adopted in February 2004 and may soon enter into force. The International Maritime Organization (IMO) has produced guidelines to assess the efficacy and reliability of Ballast Water Treatment Systems (BWTS), but no guidance on how to take temperature into account during test cycles has been provided yet. Temperature is one of the main factors influencing the distribution and ecology of organisms along latitudes. Its increase results in higher grazing, growth, and reproduction rates of zooplankton. Under dark conditions, phytoplankton loss is also increased due to faster natural decay as well as enhanced top down control from zooplankton. Increased temperatures also improve the efficacy of chemical treatment, whereas the decay rates of disinfectants and their byproducts are potentially accelerated. The IMO guidelines for the type approval of BWTS should be amended to include recommendations on how to take temperature into account. Failing to ensure comparability and reliability between tests may pose a threat to the environment and may create problems for those attempting to apply BWTS. We propose to use a fixed Q10 value and a temperature of reference to adjust the retention time in ballast water tanks during testing.

Do Acartia tonsa (Dana) eggs regulate their volume and osmolality as salinity changes?

Subitaneous eggs from an euryhaline calanoid copepod Acartia tonsa were challenged by changes in salinity within the range from full strength salinity, down to zero and up to >70 psu. Egg volume changed immediately, increasing from 2.8 × 10(5) µm(3) at full strength salinity (35 psu) to 3.8 × 10(5) µm(3) at 0 psu and back to its initial volume when gradually being returned to full strength salinity. Egg osmolality followed the molality of the surrounding water when challenged within a salinity range from 2 to 50 psu. Egg respiration was not affected when eggs kept at 35 psu was exposed to low salinity (2 psu). These results suggest that eggs are unable to regulate their volume or osmolality when challenged with changes in salinity. Gradual changes in salinity from 35 to 2 psu and back did not harm the eggs (embryos), since the hatching success remained unaffected by such changes in salinity. In contrast, extreme hyper-saline conditions (76 psu) made the eggs implode and killed the embryo. We propose that the embryo is protected from salinity stress by its plasma membrane and that water exchange driven by osmosis is restricted to the perivitelline space of the egg, which acts as a perfect osmometer in the salinity range of 5-35 psu. We hypothesize further that the embryo is able to keep its volume and osmolality constant due to the impermeability of the inner plasma membrane of the egg or by a combination of osmoregulation and reduced permeability of the inner plasma membrane.

Do inactivated microbial preparations improve life history traits of the copepod Acartia tonsa?

We have tested a microbial preparation with probiotic effects (PSI; Sorbial A/S DANISCO) on the calanoid copepod Acartia tonsa (Dana) development time and reproduction effectiveness in culture. The hypotheses were that PSI increases the productivity and quality of copepods in culture (increased egg production and hatching success, HS). This was carried out because the use of copepods as live prey in aquaculture could increase the number of fish successfully raised through their entire life cycle. However, the availability of copepods is limited by their difficulty to be effectively raised. Our results show that the addition of PSI to the algal food increased the individual size of the adult females and their egg production. The PSI, together with Rhodomonas salina, also increased the HS of the eggs produced by PSI-treated females. These effects were observed despite that the biochemical analysis of the PSI revealed that it is a nutritionally poor food lacking essential fatty acids, and hence it cannot be used alone to raise copepods but instead as a food additive. This is the first demonstration that the effectiveness of copepod culturing can be improved using microbial preparations as a food additive.