Responses of Estuarine Bacterioplankton, Phytoplankton and Zooplankton to Dissolved Organic Carbon (DOC) and Inorganic Nutrient Additions

Abstract

The response of planktonic bacteria and phytoplankton to various additions of dissolved organic carbon (DOC) as glucose, with and without inorganic nutrients (nitrogen and phosphorus), was tested in the upper to mid Hunter Estuary, Australia. In situ microcosms (1.25 L) were performed at two sites with varying salinities over three seasons. Analysis of variance showed a significant difference among control and treatments for all seasons for the bacterial, dissolved oxygen and chlorophyll a responses (P<0.05). A significant interaction between treatment and site was found in autumn for dissolved oxygen, autumn and spring for bacterial and spring for chlorophyll a responses. At both sites for each season, and on nearly all occasions, bacterial surface area was enhanced by DOC addition as indicated by both increased bacterial abundance and dissolved oxygen utilisation. DOC in combination with inorganic nutrients sometimes further enhanced the bacterial response compared to DOC alone. Inorganic nutrients alone did not enhance growth of the heterotrophic bacterioplankton. Addition of DOC alone led to decreased chlorophyll a relative to the control, probably due to competition for limited inorganic nutrients with the bacterioplankton DOC non-limiting conditions. Results suggest that the heterotrophic community was limited by DOC at both sites and across seasons. An experiment with a larger volume (70 L), performed over a longer time, compared a control with DOC addition. Increased bacterial biomass as a result of DOC addition occurred at day 2. Chlorophyll a did not significantly differ between treatments. An increase in zooplankton density was recorded in the DOC treatment relative to the control at day 10. This study supports the contention that increased DOC delivery with river inflows through environmental flow allocations will stimulate heterotrophic bacterioplankton production in the upper Hunter Estuary.

Zooplankton response to organic carbon level in lakes of differing trophic states

“…Responses of subsequent trophic levels are thus expected, as has been suggested by other studies showing that increases in nutrient availability evoke a reaction from organisms suspended in the water column, thereby influencing carbon flux in this part of the ecosystem (Olsen et al, 2007). Depending on the level of nutrients, the addition of DOC into the environment stimulates planktonic bacteria or phytoplankton and thus alters the response of zooplankton (Hitchcock et al, 2010). Limited attempts have been made to relate the zooplankton community to the organic matter content in lakes along a trophic gradient.…”

“…Dissolved organic carbon is closely correlated with communities and directly or indirectly influences aquatic organisms and their habitats. Numerous studies have demonstrated a positive correlation between bacteria abundance and DOC concentration; DOC constitutes an alternative source of carbon and energy for zooplankton (Azam et al, 1983;Jansson et al, 1999;Chróst and Siuda, 2006;Hitchcock et al, 2010). On the other hand, by affecting water color, increasing concentrations of DOC in lake water may limit the extent of the euphotic zone and primary production rate, even though DOC is subject to partial photodegradation under sunlight (Perez-Fuentetaja et al, 1999;de Lange et al, 2003).…”

River bacterioplankton community responses to a high inflow event

“…Recent studies within Eastern Australian coastal river and estuarine freshwater inflows have measured patterns in bacterial abundance and biomass, and have revealed a common positive correlation between the import of allochthonous DOC and inorganic nutrients and these parameters (Hitchcock et al 2010, Hitchcock & Mitrovic 2013. We have expanded upon these observations by identifying shifts in BCC and taxonomic diversity in response to an inflow event and have related these changes to the input of allochthonous C and inorganic nutrients.…”

“…Previous studies have performed nutrient manipulation experiments using microcosms to investigate heterotrophic bacterial growth response to simulated inflows, floods and their associated nutrient inputs (Hitchcock et al 2010, Hitchcock & Mitrovic 2013). These studies have indicated that heterotrophic bacterial growth in rivers and estuaries is often limited or co-limited by available DOC, N and P during periods of low river discharge (Jansson et al 2006, Hadwen et al 2010, Hitchcock et al 2010, Hitchcock & Mitrovic 2013), but very little is known about how these events influence the composition and diversity of bacterial assemblages, or the implications for lotic chemical cycling and trophic dynamics.…”

“…These studies have indicated that heterotrophic bacterial growth in rivers and estuaries is often limited or co-limited by available DOC, N and P during periods of low river discharge (Jansson et al 2006, Hadwen et al 2010, Hitchcock et al 2010, Hitchcock & Mitrovic 2013), but very little is known about how these events influence the composition and diversity of bacterial assemblages, or the implications for lotic chemical cycling and trophic dynamics. This knowledge is of growing importance because there is a global pattern of high flow events being returned to many major coastal rivers and estuaries as a result of flow management (Dudgeon 2010).…”