Do rivermouths alter nutrient and seston delivery to the nearshore?

Abstract

Summary Tributary inputs to lakes and seas are often measured at riverine gages, upstream of lentic influence. Between these riverine gages and the nearshore zones of large waterbodies lie rivermouths, which may retain, transform and contribute materials to the nearshore zone. However, the magnitude and timing of these rivermouth effects have rarely been measured. During the summer of 2011, 23 tributary systems of the Laurentian Great Lakes were sampled from river to nearshore for dissolved and particulate carbon (C), nitrogen (N) and phosphorus (P) concentrations, as well as bulk seston and chlorophyll a concentrations. Three locations per system were sampled: in the upstream river, in the nearshore zone and at the outflow from the rivermouth to the lake. Using stable oxygen isotopes, a water‐mixing model was developed to estimate the nutrient concentration that would occur at the rivermouth if mixing was strictly conservative (i.e. if no processing occurred within the rivermouth). Deviations between these conservative mixing estimates and measured nutrient concentrations were identified as rivermouth effects on nutrient concentrations. Rivermouths had higher concentration of C and P than nearshore areas and more chlorophyll a than upstream river waters. Compared to the conservative mixing model, rivermouths as a class appeared to be summer‐time sources of N, P and chlorophyll a. Substantial among rivermouth variation occurred both in the effect size and direction for all constituents. Using principal component analysis, two groups of rivermouths were identified: rivermouths that had a large effect on most constituents and those that had very little effect on any of the measured constituents. ‘High‐effect’ rivermouths had more abundant upstream croplands, which were presumably the sources of inorganic nutrients. Cross‐validated models built using characteristics of the rivermouth were not good predictors of variation in rivermouth effects on most constituents. For consumers feeding on seston and microbes and vascular autotrophs directly taking up dissolved nutrients, rivermouths are more resource‐rich than upstream riverine or nearby Great Lakes waters. Given declines over time in open‐lake productivity within the Great Lakes, rivermouths may contribute more productivity than their size would suggest to the Great Lakes food web.

Riverine transport and nutrient inputs affect phytoplankton communities in a coastal embayment

“…This is consistent with other studies that report phytoplankton transport from rivers to embayments (Larson et al, 2016). The riverine transport of phytoplankton into the bay was clearly important.…”

“…Larson et al, 2016) , on the assumption that the model implies a continuous distribution between two finite and non-zero values if the percentage of river water never goes to zero. Larson et al, 2016) , on the assumption that the model implies a continuous distribution between two finite and non-zero values if the percentage of river water never goes to zero.…”

“…Coastal embayments and river-mouth ecosystems are economically and ecologically important regions (Braden et al, 2008;Reichert et al, 2010) that provide numerous ecosystem services (Larson et al, 2013;Sebastiá & Rodilla, 2013;Larson, Frost, Vallazza, Nelson, & Richardson, 2016), and may be early warning systems for the larger lake system and lakes in general (Abirhire, North, Hunter, Vandergucht, & Hudson, 2016;Spatharis, Tsirtsis, Danielidis, Chi, & Mouillot, 2007). Coastal embayments are also vulnerable to excessive nutrient loading and eutrophication because they are often sheltered regions with longer residence times (Hall et al, 2003).…”

“…Rivers may affect embayments directly by transporting algal species and/or indirectly by providing nutrients, which may in turn lead to algal growth (Bridgeman et al, 2012;Johengen, Biddanda, & Cotner, 2008;Larson et al, 2016) and gradients in productivity between the nearshore and offshore regions (Pavlac et al, 2012).…”

Phosphorus Dynamics and Availability in the Nearshore of Eastern Lake Erie: Insights From Oxygen Isotope Ratios of Phosphate

“…Previous studies of the lower Grand have indicated that an appreciable portion of dissolved P is converted into phytoplankton biomass prior to entering Lake Erie (Kuntz, 2008). Despite the overall eutrophic conditions, the increasing disequilibrium from δ 18 O P_EQ at the river mouth during the summer in combination with high chlorophyll a concentrations (25-48 µg L −1 ) is not inconsistent with a highly productive river estuary where even the use of DOP can be important (Larson et al, 2016;Yuan et al, 2017). Further study of phosphorus sources to the lower river and speciation would be required to confirm such pathways, particularly in order to trace back to specific input sources of P. Nonetheless, even though riverine δ 18 O P appears to be altered prior to discharge into the lake, when our δ 18 O P data are considered together with evidence of strongly non-conservative mixing behavior of SRP, we infer that most of the P i discharged by the river is quickly assimilated into phytoplankton and rapidly overprinted via DOP recycling.…”