Consequences of Hypoxia on Estuarine Ecosystem Function: Energy Diversion From Consumers to Microbes

Abstract

As in other eutrophied estuaries and coastal embayments, persistent hypoxia now routinely develops during summer in the mesohaline portion of the Neuse River estuary (North Carolina, USA). In response to interannual differences in hydrography, summer 1997 exhibited much more intense and widespread hypoxia than summer 1998, permitting inferences about impacts of hypoxia on food web dynamics by comparing system changes across these two summers. The trophic structure of the Neuse estuary now resembles the generic pattern for a degraded temperate estuary with (1) intense planktonic algal blooms and similarly high production of free-living bacteria, (2) trivial levels of abundance of rooted aquatic plants and benthic macroalgae, (3) depleted apex predators, and (4) functional extinction of the historically dominant benthic grazer, eastern oysters. Detailed carbon-flow models, based on comprehensive field data, demonstrated large differences between the two summers in trophic transfers and system dynamics. Largely because of greater mortality of benthic invertebrates from more intense hypoxia, total biomass of heterotrophs declined over summer by 51% in 1997 as compared to only 17% in 1998. Because net primary production increased over summer and herbivory in this system is predominantly benthic, the fraction of primary production consumed by herbivores declined over summer by 35% in 1997 and 29% in 1998. Influx of juvenile fishes and their rapid growth in the estuarine nursery over summer led to increases in energy demand by demersal fishes of 380% and 507% in the successive summers. Thus, hypoxia-enhanced diversion of energy flows into microbial pathways away from consumers and mass mortality of benthic invertebrates from bottom hypoxia occurred at the season of greatest demand by predatory fishes and crabs using the estuary as nursery. Average residence time of carbon in the ecosystem declined by 51% in 1997 and 29% in 1998. Total system throughput declined over summer 1997 while increasing in 1998, indicating the reduced capacity of the system to transfer carbon to higher trophic levels in the more hypoxic summer. Late-summer trophic pathways were characterized by greater numbers of cycles, but flows became increasingly dominated by microbial loops rather than transfers to consumers. Ecosystem trophic efficiency was only ϳ4%, lower than other estuaries similarly analyzed. System properties indicative of resiliency of system function including development capacity, ascendancy, and flow diversity declined over summer 1997, while increasing or declining less in 1998. Thus, intensification of hypoxia caused dramatic reduction in the ecosystem's ability to transfer energy to higher trophic levels and rendered the ecosystem potentially less resilient to other stressors.

Effect of invasive species on the structure and function of the Sylt-Rømø Bight ecosystem, northern Wadden Sea, over three time periods

“…This assumption implies that the ecosystem as a whole also occurs in a steady-state and assumes the underlying principle of the conservation of mass, namely that total inputs are balan ced by total outputs. The mass-balancing of trophic networks, however, has limitations and weaknesses (Menge 1995, Baird et al 2004b), while Baird et al (2009) commented on the consequences of the balancing of networks on the properties derived from ENA. Here we present results from network analysis based on the steady states of 3 models.…”

“…The Finn cycling index (or FCI) is derived from the fraction of the sum of flows that is devoted to cycling, and is equal to Tc/TST, where Tc is the amount recycled and TST (total systems throughput) is the sum of all flows in the ecosystem. The FCI is an index of the retentiveness of the system (Baird & Ulanowicz 1989, Baird et al 2004b.…”

Variability in Fish Tissue Proximate Composition is Consistent with Indirect Effects of Hypoxia in Chesapeake Bay Tributaries

“…The intensity and duration of hypoxia directly affect benthic productivity (Baird et al 2004;Long and Seitz 2009;Sturdivant et al 2013), and the single best predictor of benthic organism density and diversity in Chesapeake Bay is DO (by depth), which outperforms other important habitat characteristics such as sediment type and salinity ). Hypoxia lasting only a few hours to a few days increases the transfer of trophic energy to higher trophic levels (e.g., fish), whereas longer-lasting hypoxia results in the loss of benthic production and a shift toward microbial production (Baird et al 2004;Powers et al 2005;Long and Seitz 2009). The 2011 hypoxic event that persisted for more than 10 weeks in the Rappahannock River likely disrupted the system's normal benthopelagic food web.…”

The filtering capacity of selected Eastern Cape estuaries, South Africa

“…Conley et al, 1993;Paerl, 1997;Howarth, 1998). The occurrence of hypoxia appears to be a common phenomenon during late summer in many estuaries along the eastern seaboard of the continental United States (Baird et al, 2004). Eutrophication is also associated with changes in the food web structure, e.g.…”

“…increased phytoplankton biomass at the expense of macrophytes, or the shift from edible to inedible phytoplankton species and the consequences for higher trophic levels (e.g. Nixon, 1993;Roelke et al, 1999, Baird et al, 2004. The overall nutrient loads reaching South African estuaries is still only a small fraction compared to those of Europe and North America.…”