Abstract
Phytoplankton biomass is commonly used as a water quality metric in the management of anthropogenic nitrogen and phosphorus loading, yet interpretation of the long‐term response of phytoplankton biomass to nutrient regulation may be confounded by changes in zooplankton grazing pressure. Zooplankton community structure may be affected by planktivorous fish whose populations are subject to losses through fishing and gains through restoration efforts. We investigated temporal changes in phytoplankton biomass, nitrogen and phosphorus concentrations, zooplankton, and planktivorous fish in the tidal fresh and oligohaline Chowan River, NC, over a 30‐year period in order to compare long‐term trends in these parameters and infer their relative influence on phytoplankton biomass. Data were compiled from the records of two state agencies and several academic studies and supplemented by our own two year monitoring of water quality and zooplankton in the Chowan River mainstem and two tributaries. Seasonal trend decomposition using locally weighted regression was applied to chlorophyll a (a proxy for phytoplankton biomass), nitrogen, and phosphorus, and showed that chlorophyll a declined more quickly than did nutrient concentrations over the 30‐year period. Despite the long‐term decline in nitrogen and to a lesser extent phosphorus, the long‐term trend in phytoplankton growth rate (predicted using an empirical model involving irradiance and nutrients) remained relatively constant. Zooplankton abundance increased from the period 1981–1982 to 2008–2010, as did the predicted zooplankton community water clearance rate (a proxy for zooplankton grazing). River herring, historically the dominant planktivore, declined dramatically over the 30‐year period. The data indicate that the most parsimonious explanation for the long term decrease in chlorophyll a is an increase in zooplankton and not a decrease in nutrients. This inference leads us to hypothesize that the historically large river herring population exerted a positive, indirect influence on phytoplankton biomass in the Chowan River system, and that the decline in river herring has lead to higher zooplankton abundance and subsequently lower phytoplankton biomass.
Highlights
Syntheses of 3–4 decades of environmental data in estuaries is fostering an appreciation for the complexity of estuarine food webs and paving the way toward new hypotheses (Kemp and Boynton 2011)
Our study addresses two questions: First, how has the change in anthropogenic nutrient inputs over the past 30 years affected phytoplankton growth in the Chowan River? We use an empirical model of phytoplankton growth parameterized with longterm data on nutrient concentrations and other environmental and physiological variables to answer this question
If not due to a reduction in nutrient concentrations, what explains the reduction in chlorophyll a during the early 1980s? One explanation for the decline in phytoplankton biomass in the Chowan River may be a shift in phytoplankton community away from highly productive, bloom-forming species with a high concentration of chlorophyll a per unit biomass
Summary
Syntheses of 3–4 decades of environmental data in estuaries is fostering an appreciation for the complexity of estuarine food webs and paving the way toward new hypotheses (Kemp and Boynton 2011). The emergent effects of trophic interactions and resource limitation are being discovered in analyses of long time-series data, where the importance of indirect trophic effects (e.g., Winder and Jassby 2010) and environmental chemistry (e.g., Glibert 2010) become apparent. It is possible that long term declines in anadromous planktivores may indirectly suppress phytoplankton biomass, a phenomenon observed in lakes (Post et al 2008) This indirect effect of zooplanktivorous fish on phytoplankton might obscure interpretation of phytoplankton trends that would otherwise be attributed to restrictions on nutrient inputs. Restoration of anadromous fish populations may restore a trophic cascade that indirectly enhances phytoplankton biomass
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
