Mesocosm research on the effects of nutrient limitation on periphyton quality

TROCHINE CAROLINA; CUASSOLO FLORENCIA; URSULA STRANDBERG; THOMAS DAVIDSON ; JEPPESEN ERICK

Simposio; 2ND INTERNATIONAL AQUATIC MESOCOSM RESEARCH SYMPOSIUM; 2021

Periphyton is a key contributor to primary production, particularly in environments with a developed littoral area. The quality of periphyton, however, may differ under differing nutrient and warming conditions. Particularly, omega-3 long-chain polyunsaturated fatty acids EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are markers of nutritional quality, as they are physiologically essential biomolecules for consumer?s growth and reproduction. Besides, these FA are mostly coming from the diet. The objective of the present study was to elucidate how nutrient limitation affects periphyton quality (measured as EPA+DHA content) in eutrophic systems with contrasting temperatures - before and after a major change in external N loading. We performed the investigation at AU Lake Mesocosm Warming Experiment (LMWE): 24 mesocosms mimicking shallow lakes with two nutrient treatments (unenriched and enriched [weekly amended with P and N]) and three temperatures (A0, ambient, A2, IPCC A2 and A2+, IPCC A2+50%). In the enriched mesocosms, the external N-loading was stopped in June 2018 for one year (only external P loading continue). N-loading was re-started in June 2019. We added nutrient diffusion substrata (NDS, 2% agar alone or releasing N, P and combined N and P with a GF-F filter on top) to these 12 mesocosms on two occasions, before and after returning to the normal high N-loading. The NDS were kept in the mesocosms for 21 days to allow colonization by periphyton. We then assessed periphyton EPA+DHA (µg/mg DW) content.The external N loading did not significantly change the quality of periphyton. However, we found that nutrient supply through the NDS (+P, +N, +NP) affects the quality of the periphyton. The addition of nutrients showed differential effects on the EPA+DHA content of periphyton that were further related to the temperature scenarios. The EPA+DHA content of periphyton was not significantly affected by P supply through the NDS in any temperature scenario. However, temperature affected periphyton EPA+DHA content when combined with N availability; the lowest values were found in response to N supply (+N) in the warmest temperature scenario (A2+). We related these changes to specific fatty acids markers that identify changes in periphyton community composition: rich EPA+DHA taxa as e.g. diatoms we more abundant in the control and +P treatments, while +N and +NP treatments evidenced higher abundances of low quality algae, e.g., cyanobacteria. These results have implications in the food web perspective, which we plan to explore in detail through a follow up AQUACOSM TA project design to investigate the impact of nutritional changes in periphyton to the FA composition of primary consumers.