Response of chlorophyll a to total nitrogen and total phosphorus concentrations in lotic ecosystems: a systematic review

Abstract

Background Eutrophication of freshwater ecosystems resulting from nitrogen and phosphorus pollution is a major environmental stressor across the globe. In this systematic review, we compiled and synthesized literature on sestonic and benthic chlorophyll a (chl-a) responses to total nitrogen (TN) and total phosphorus (TP) concentrations in the water column in streams and rivers to provide a state-of-the-science summary of nutrient impacts on these endpoints. This review was motivated by the need for comprehensive information on stressor-response relationships for the most common nutrient and biotic response measures used by state-level environmental managers in the United States to assess eutrophication of lotic ecosystems and support environmental decision making. Methods Searches for peer-reviewed and non-peer-reviewed articles were conducted using bibliographic databases, specialist websites, and search engines. These returns were supplemented with citation mapping and requests for material from experts. Articles were screened for relevance using pre-determined eligibility criteria, and risk of bias was evaluated for each included article based on study type-specific criteria. Narrative summaries and meta-analysis were used to evaluate four primary stressor-response relationships: TN-benthic chl-a, TP-benthic chl-a, TN-sestonic chl-a, and TP-sestonic chl-a. Potential effects of modifying factors and study validity on review conclusions were assessed via sensitivity and sub-group analysis and meta-regression. Results Meta-analysis of 105 articles, representing 439 cause-effect pairs, showed that mean effect sizes of both benthic and sestonic chl-a responses to TN and TP were positive. Of the four stressor-response relationships examined, TP-sestonic chl-a had the most positive relationship, followed by TN-benthic chl-a, TN-sestonic chl-a, and TP-benthic chl-a. For individual U.S. states, mean effect sizes for the four stressor-response relationships were mostly positive, with a few exceptions. Chlorophyll measurement method had a moderately significant influence on mean effect size for TP-sestonic chl-a, with chl-a responding more strongly to TP if fluorometry versus spectrophotometry was used. Year of publication had a significant negative effect on mean effect size, as did mean nutrient concentration for both sestonic chl-a nutrient relationships. When the same study measured both TN and TP, chl-a tended to respond similarly to both nutrients. Sensitivity analysis indicated that conclusions are robust to studies with high risk of bias. Conclusions This systematic review confirms that nutrients consistently impact primary producer biomass in streams and rivers worldwide. It builds on previous literature syntheses evaluating chl-a responses to nutrient concentrations and confirms that benthic and sestonic chl-a respond positively to nutrients across a range of stream and river conditions, but also points to limits on these relationships (e.g., potential saturation at high nutrient concentrations). Lack of consistent reporting of contextual data limited our ability to examine how moderating factors influenced these stressor-response relationships. Overall, we provide nutrient managers responsible for protecting the quality of lotic ecosystems with a comprehensive evidence base for chl-a responses to TN and TP concentrations in the water column.

Ecosystem‐level effects of re‐oligotrophication and N:P imbalances in rivers and estuaries on a global scale

“…One of the most comprehensive available meta-analysis of world rivers showed positive responses of Chl a in the water column to TP and N (Bennett et al, 2021) However, those databases rarely include estuaries; in most cases datasets from estuarine systems come from local or regional studies. Furthermore, most of the studies encompasses one visit at a year and focuses on few spatial stations across years (Ardón et al, 2021;Bennett et al, 2021). This make difficult to elucidate if declining P trends are linked to biological responses.…”

“…Regarding the response of river biota to P decline, the most basic and available variable is Chl (either total or a ). Although existing datasets are quite limited across countries and along time (Bennett et al, 2021 ), there is much more data than for other variables such as phytoplankton biomass (or biovolume), phytoplankton composition, macrophyte biomass, etc. (in those cases data are very scarce).…”

“…cascading effects, chlorophyll, ecosystem shift, oligotrophication, phosphorus, primary producers, running waters, stoichiometry heterogeneous chlorophyll (Chl) observations as well as lack of contextual information, often results in inconsistent findings of nutrient-Chl relationships in the literature (Bennett et al, 2021). There is an urgent need to place individual case studies into a broader context and general comprehensive evidence based on available literature to outline how ongoing and future water management and restoration can benefit from a better knowledge of the consequences of re-oligotrophication process.…”

“…After extensive research in lakes, there also has been efforts to document fluvial and estuarine ecosystem responses to re‐oligotrophication (Duarte et al, 2009 ; Hilt et al, 2011 ). Now, and despite a broad understanding on water quality trajectories and environmental controls (Dent et al, 2002 ; Ibáñez, Alcaraz, et al, 2012 ) and more recently on the main ecological responses (Bennett et al, 2021 ; Diamond et al, 2022 ), the timing, location, and magnitude of river and estuary re‐oligotrophication, its nutrient‐biological response relationships and potential drivers remain largely unknown.…”

“…For instance, the top‐down effect of fish and bivalves (e.g., control of phytoplankton via grazing; Minaudo et al, 2021 ), the bottom‐up effect (e.g., control via nutrient limitation; Ibáñez, Alcaraz, et al, 2012 ), or legacy nutrient sources from historical agricultural sediments (Civan et al, 2018 ). This complexity, coupled with the fact that long‐term (>20 years) river datasets suffer from heterogeneous chlorophyll (Chl) observations as well as lack of contextual information, often results in inconsistent findings of nutrient‐Chl relationships in the literature (Bennett et al, 2021 ). There is an urgent need to place individual case studies into a broader context and general comprehensive evidence based on available literature to outline how ongoing and future water management and restoration can benefit from a better knowledge of the consequences of re‐oligotrophication process.…”