Changes in primary productivity in Sydney Harbour following a storm event: using cell-size as a descriptor

Estuaries are highly valued ecosystems where land-sea interactions cause significant variation in physicochemical characteristics. Seasonal inputs of freshwater (high in nutrients) can trigger phytoplankton blooms – the base... [ view full abstract ]

Estuaries are highly valued ecosystems where land-sea interactions cause significant variation in physicochemical characteristics. Seasonal inputs of freshwater (high in nutrients) can trigger phytoplankton blooms – the base of the foodweb in these environments. Estuarine ecosystems are also often heavily urbanised with a legacy of contaminants and ongoing stressor inputs. Human pressure in estuaries has been shown to alter interactions between the biological and physicochemical components, with increased intensity and reduced frequency of rain events likely exacerbating these effects. In this study, we investigated how anthropogenic inputs from stormwater runoff could influence processes in an urbanised system with a legacy of heavy metal contamination. Size-fractionated (0.2-3 µm, 3-10 µm, and >10 µm) primary productivity (PP) was measured in estuarine surface waters at increasing temporal intervals (~48 h, 8 d, 21 d, and 60 d post rainfall; PRF) following a rainfall event (>100 mm in 24 h) in Sydney Harbour.
Four locations with large points of discharge into the estuary were sampled. To investigate whether the residence period for the stormwater runoff would influence biogeochemical processes we selected two locations that were poorly flushed embayments and two well-flushed channels. Samples were collected at distances of 0, 200 and 1,000 m from the points of discharge.
PP for the whole phytoplankton population showed no patterns associated with time or sampling distance. PP ranged between 115±3 to 1,140±40 mg C m-3 d-1 (mean±SE) with the exception of one of the channel locations (2,970±90 mg C m-3 d-1; Parramatta River, 48 h PRF). However, there was a consistent shift in the PP of size-classes following rainfall. PP increased by 40-80 % in the largest size-class 48h after the rain event, with the other size-classes showing a smaller contribution. Eights days PRF the inverse trend was observed, with the proportion of the large size-class being <20 % at all locations. These size-related shifts of carbon production can directly impact biogeochemical cycles and food webs. Chlorophyll a, light and nutrient data will also be presented to further explain how strongly size-class shifts in production can be related to environmental changes (physicochemical and/or biological interactions).