A shift in the pool of retained microphytobenthos nitrogen under enhanced nutrient availability

Area of Science:

• Marine Ecology
• Biogeochemistry
• Microbial Ecology

Background:

• Sediment microbial communities are key sinks for nitrogen (N).
• Microphytobenthos (MPB) biomass significantly contributes to short-term N assimilation and retention.
• The impact of anthropogenic nutrient enrichment on MPB-derived N (MPB-N) processing remains unclear.

Purpose of the Study:

• To investigate how nutrient enrichment affects the microbial assimilation, processing, and fate of MPB-derived N.
• To quantify changes in MPB-N flux pathways under varying nutrient concentrations.

Main Methods:

• In situ labeling of an MPB-dominated microbial community with ¹⁵NH₄⁺-N.
• Laboratory core incubations under four nutrient enrichment levels (ambient to 10× ambient).
• Tracking of ¹⁵N-labeled MPB-N fate over 10.5 days.

Main Results:

• Nutrient addition stimulated short-term retention of MPB-N by MPB (71-93% vs. 38% at 0.5 days).
• After 10.5 days, nutrient-enriched treatments showed increased MPB-N turnover into uncharacterized organic N (45-75%).
• Reduced DON efflux (3.9-5.2% vs. 10.9%) and small water column exports (max 14%) were observed under enrichment.

Conclusions:

• Increased nutrient loading enhances MPB-N retention but shifts N from MPB biomass to other sediment organic N pools.
• Preferential uptake of inorganic nutrients by microbes under enrichment reduces MPB-N recycling and DON efflux.
• Denitrification and DON export are minor pathways for MPB-N loss from the sediment.