Grazing And Assimilation Of Phytoplankton Blooms

The assimilation of eutrophication is an under-appreciated management consideration for maintaining water quality. The invasion of the Great Lakes in the north-eastern US by the zebra mussel (Dreissena polymorpha) has fundamentally altered the ecology of those lakes. By filtering out the lakes' phytoplankton, zooplankton populations have collapsed and so have the zooplanktivorous fish (such as the 'alewife' Alosa pseudoharengus, which was also introduced). Zebra mussels are also found to decimate the phyto-plankton concentration in Hudson River and San Francisco Bay. Recently, the pygmy mussel Xenostrobus securis has been implicated in the rapid demise of phytoplankton blooms in the Wallamba River (central coast of New South Wales, Moore et al. 2006). Xenostrobus aggregates on the mangrove aerial roots in brackish waters. Up to 25% of the decline in phy-toplankton blooms was attributed to the pygmy mussel, but the remaining 75% (unrelated to hydrography) could be caused by zooplankton or population decay by salinity stress (Moore et al. 2006).

Zooplankton can reduce the frequency of harmful algal blooms by keeping bloom species at low concentrations via grazing (Chan et al., 2006), and the zooplankton biomass can increase. Analysing sufficient zooplankton samples to understand the interactions taking place in estuaries can be time consuming and answers can be achieved more rapidly by using a particle counting and sizing device. The abundance of various size categories of zooplankton can yield a useful estimate of grazing and production rates, because metabolic rate is predictably related to body size (Section 2.1). Biomass is passed from smaller to larger particles via predation (Figure 3.3). Particle size is measured by an optical plankton counter or image analysis as area, which is converted to biomass assuming a density of water and the volume of a sphere (see Section 4.9). The slope of the NBSS is theoretically around -1 (Figure 3.3), which serves as an index of zooplankton production, although the interpretation is complicated by both top-down (predation) and bottom-up (nutrient) effects.

To assess the effect of catchments on zooplankton, we determined the size frequency distribution of zooplankton in three contrasting NSW estuaries using an optical plankton counter (Moore and Suthers 2006). One

Timel

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a) nutrien^v pulse

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b) sustained^v nutrient suppl^\

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same hi intercept

\c) size-selective predation by fish

excretion?\ \

hi slope \ \hi intercept

Size

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Figure 3.3 Sketch of possible bottom-up and top-down processes altering the -1 slope and intercept of the zooplankton NBSS (Normalised Biomass Size Spectrum) around Cato Reef, during three time periods. a) A nutrient pulse stimulates phytoplankton and increasing the (normalised) biomass concentration of small zooplankton particles, which is passed by predation to larger particles. b) A sustained nutrient supply increases the biomass and intercept. c) Size-selective predation by larval and juvenile fish could steepen the slope, and their excreted nutrients could increase the production of smaller particles (adapted from Suthers et al. 2006).

estuary had a forested and less-developed catchment (the Wallingat River) while the other two estuaries had catchments dominated by dairy farming and hence had enhanced nutrient flows. Zooplankton was collected by towing a 100 ^m mesh net at replicated stations. We found the monthly variation was related to rainfall and nutrient supply to the estuaries. There were significant differences in the zooplankton NBSS between large

Log]() body mass (mg) Figure 3.4 The average Normalised Biomass Size Spectrum (NBSS) for zooplankton caught in a 100 pm mesh net in three temperate estuaries, during four summer months (after Moore and Suthers 2006).

estuaries with rural catchments and nutrient enrichment, versus the small estuary with a forested catchment (Figure 3.4). The more pristine estuary often had a steeper slope and lower overall biomass, which we attribute to the greater water clarity allowing visual-feeders such as fish to predate the larger zooplankton and thus steepen the slope (Figure 3.3).

The role that zooplankton play in assimilating algal biomass was shown clearly in work conducted in Dee Why lagoon - a small coastal lake in the northern beaches area of Sydney. The lake is closed off from the ocean for long periods of time, which removes the influence of tidal flushing and enables biological responses to rainfall to be examined. We sampled nutrients, phytoplankton and zooplankton at regular intervals before and after a large rainfall event, after a prolonged summer dry period. Nutrients (ammonia and oxidised nitrogen) significantly increased the day after initial rainfall, before returning to pre-rainfall conditions within 5 days. In response, phytoplankton a) chlorophyll re

T3 C

a) chlorophyll re

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Figure 3.5 Changes to average plankton at two sites within Dee Why lagoon over the study period. Vertical dashed lines indicate the main, initial rain event. a) Phytoplankton biomass (pg chl-a.L"1). b) Oithona, an adult copepod, which doubled in abundance within 48 hours. c) Copepod nauplii. d) Adult Acartia bispinosa.

80000

60000

T3 C

20000-

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Figure 3.5 Changes to average plankton at two sites within Dee Why lagoon over the study period. Vertical dashed lines indicate the main, initial rain event. a) Phytoplankton biomass (pg chl-a.L"1). b) Oithona, an adult copepod, which doubled in abundance within 48 hours. c) Copepod nauplii. d) Adult Acartia bispinosa.

biomass grew 10-fold within a week after the initial rainfall and declined to near original levels 2 weeks later (Figure 3.5a). Blooms of diatoms followed the rainfall within a week, which returned to pre-rainfall levels within 2 weeks. It was clear that zooplankton, which increased in response to the higher phytoplankton concentrations, were responsible for the rapid decline in phytoplankton. However, some zooplankton responded within a day with two fold increase in the adult stages of the calanoid copepod Oithona sp. (Figure 3.5b), followed a week later by nauplii (Figure 3.5c) and adult Acartia bispinosa (Figure 3.5d). The influx of adult zooplankton into the water column was presumably from resting populations that were previously under sampled by our plankton net. The zooplankton community returned to the initial state by 2 weeks and then matured to a centric diatom-Acartia dominated population after 5 weeks.

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