Ammonium cycling in the rocky intertidal: Remineralization, removal, and retention

Abstract

Rocky intertidal productivity is traditionally thought to be sustained almost solely by upwelled nitrate with remineralized forms of minor importance. Using tidepools as natural experimental mesocosms, we conducted 15 N-tracer experiments to test whether ammonium remineralized within the rocky intertidal is also a significant source of fixed N to localized ecosystem production. Comparison of tidepools with and without the dominant bivalve Mytilus californianus allowed consideration of its role in NH z 4 cycling. Closed water-incubation bottles were used to investigate the contribution of suspended microbes to NH z 4 cycling. Tidepools with mussels had both greater NH z 4 remineralization (two times) and NH z 4 removal as compared with those without, with daytime rates greater than nighttime rates. Incorporation of 15 NH z 4 tracer by particulate organic matter and macroalgae, and the persistence of this signal in tidepools for several days following the experiment, showed retention of autochthonous NH z 4 in the system. Remineralization rates were tightly correlated to removal rates when compared over all treatments and experiments, but NH z 4 remineralization was significantly greater than removal, suggesting a surplus available to nearshore primary producers.

Kelp beds and their local effects on seawater chemistry, productivity, and microbial communities

“…Seawater frozen in an additional set of HDPE bottles was used to measure seawater d 15 NH 4 isotopes at University of Massachusetts, Dartmouth using methods reported previously (Pather et al 2014, Pfister et al 2014a. DOC was measured from approximately 25 mL of seawater filtered into 40-mL glass, carbon-free vials and immediately frozen.…”

Enhanced microbial nitrogen transformations in association with intertidal macrobiota

“…For the final sample (T f ) after 3-5 hours of incubation, we filtered directly from the individual chamber. All nutrient concentrations were 190 analyzed at the University of Washington Marine Chemistry lab (methods from UNESCO, 1994), while isotope determinations were done at University of Massachusetts, Dartmouth using methodology for isotopic composition reported previously (Pather et al, 2014;Pfister et al, 2014Pfister et al, , 2016b. Briefly, nitrogen stable isotopes of ammonium were measured according to a modified version of the NH 4 oxidation method detailed in Zhang et al, (2007).…”

“…The mussels studied here also excrete ammonium and likely DON (Bayne and Scullard, 1977;Pather et al, 2014). Their filter feeding activities release DOC in many forms, and continually process organic matter that can be utilized by microbes (Jacobs et al, 2015).…”

“…The Second Beach site is described in Pather et al, (2014) and has tidepools at a height of 1.2 to 1.5 m above Mean Lower Low Water (MLLW), with a diversity of species (described in (Pfister, 2007;Pfister et al, 2016b). The chambers were 130 anchored into a number of these tidepools for 3-5 hours at a time during periods of low tides when the tidepools were emergent.…”

“…Isotope dilution is important and indicates ammonium remineralization by species within the chamber. We quantified ammonium remineralization in chambers with 15 NH 4 tracer using the methods of (Pather et al, 2014 …”