Premium
Massive developments of microbial mats following phytoplankton blooms in a naturally eutrophic bay: Implications for nitrogen cycling
Author(s) -
Graco Michelle,
Farías Laura,
Molina Verónica,
Gutiérrez Dimitri,
Nielsen Lars Peter
Publication year - 2001
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.2001.46.4.0821
Subject(s) - bay , eutrophication , phytoplankton , benthic zone , environmental science , upwelling , oceanography , algal bloom , nutrient , sink (geography) , pelagic zone , spring bloom , biogeochemical cycle , bloom , chlorophyll a , estuary , biogeochemistry , environmental chemistry , ecology , chemistry , geology , biology , botany , cartography , geography
Benthic nitrogen processes have received substantial attention because the release of nutrients from sediments can contribute to the requirements of pelagic primary production; their study can also give an estimation of the importance of the sediment as a source or a sink of nutrients. Concepción Bay is located in central Chile and is the largest (167.4 km 2 ) and most enclosed embayment on the Chilean coastline. The bay is characterized by a strong hydrographic variability produced by the spring/summer seasonal upwelling of Equatorial subsurface waters (ESSW), rich in nutrients (~25 µM NO 3 − ) and poor in oxygen (≪44.6 µM). The area was studied in order to understand the consequences of phytodetrital deposition and oxygen deficiency on the environment and benthic communities. The study was carried out by sampling at a single station (28‐m depth) in the inner part of the bay during winter (June 1998) and spring/summertime (November 1998 and January and March 1999). It was focused on measurements of benthic nitrogen fluxes, sulfate reduction, and denitrification rates before and after a phytoplankton bloom. Additionally, samples from the flocculent layer and from a semipurified bacterial mat were incubated under controlled oxygen conditions to determine NH 4 + production. NH 4 + exchange showed a clear seasonal pattern, with influxes during the winter (27.6 ± 4.9 mmol m −2 d −1 ) and high effluxes during the summer (36.6 and 20.8 mmol m −2 d −1 ) when the accumulation of fresh organic matter (evidenced as chlorophyll a) produced a flocculent layer over the sediments. Besides natural hypoxia of the bottom water associated with ESSW, the large input of organic matter resulted in anoxia within the sediment, as a consequence of respiration processes, and an enhancement in sulfate reduction rates (up to 200 mmol m −2 d −1 ). The flocculent layer then provided a favorable environment for the extensive development of Beggiatoa spp. mats. Overall, during the sampling period, NO 3 − was consumed at an average rate of 1.33 mmol m −2 d −1 . In the summer, denitrification appeared to be partially inhibited by the very negative redox conditions and could explain only 24% of the NO 3 − uptake by the sediment. The balance may be due to NO 3 − incorporation into Beggiatoa spp. Short incubations with these bacteria suggest that they are able to produce NH 4 + by dissimilatory NO 3 − reduction, taking advantage of their ability to store NO 3 − , though its uptake was not observed in these experiments. The NH 4 + flux obtained using Beggiatoa spp. mat cultures was 5 mmol m −2 d −1 , which accounts for 17% of the total NH 4 + efflux during the summer period (January and March). The ecological implications of a large input of organic matter, evidenced by the presence of a flocculent benthic layer and Beggiatoa spp., are discussed in relation to their contribution, during the upwelling season, toward the long‐term eutrophication of Concepcion Bay.