Primary Production, Community Respiration, and Net Community Production along Oxygen and Nutrient Gradients: Environmental Controls and Biogeochemical Feedbacks within and across “Marine Lakes”

Abstract

Declining ocean oxygen content driven by anthropogenic climate change has wide-ranging ramifications for marine ecosystems. These effects are significant but complex at the upper margins of expanding oxygen minimum zones (OMZs), where deoxygenation, and biogeochemical feedbacks to low dissolved oxygen (DO) are regulated by biological production and consumption of DO via gross primary production (GPP) and community respiration (CR). We used "marine lakes"-bodies of seawater surrounded by land-as a natural experimental system for examining the environmental factors that dictate coupling and de-coupling between GPP and CR. Distinct gradients in DO (from fully oxygenated to anoxic conditions), temperature, light, quantity, and quality of organic matter, presence of key nutrients, and microbial community structure occur across multiple stratified meromictic lakes and well-mixed holomictic marine lakes present in Palau. We found consistently high GPP rates in stratified meromictic lakes (>10 mmol O m −3 d −1 2 )-especially near the chemocline, where nutrients diffuse upwards from anoxic waters-and a wider range of rates in well-mixed lakes (0.350-57.9 mmol O m −3 − 2 d 1 ). In contrast to GPP, CR rates were typically lower and less variable across different depths and lakes. Most depths in most lakes were therefore net autotrophic [i.e., net community production (NCP) > 0]. However, experimental additions of ammonium (5 µM) and labile organic carbon (100 µM) had strong effects on CR, resulting in (i) several-fold increases in CR, (ii) larger increases in CR in meromictic lakes with chemoclines near the surface, and (iii) the occurrence of net heterotrophy and DO consumption. Our results are indicative of several biogeochemical feedback mechanisms to deoxygenation present Wilson et al. Biogeochemical Cycling in Marine Lakes at the upper margins of shoaling OMZs that are governed by nutrient turnover. In marine lakes, these feedbacks can have strong effects on nutrient uptake and the production and consumption of oxygen, with implications for carbon, nutrient, and oxygen cycling throughout large areas of the ocean.

Climatic, physical, and biogeochemical changes drive rapid oxygen loss and recovery in a marine ecosystem

“…Notably, rate magnitudes were similar between sensors and bottles (Fig. S3) and consistent with earlier work showing high rates of GPP 13,12 and CR 12 . Over multi-day sensor deployments in each lake, surface NCP values were frequently autotrophic (NCP >0) while deeper depths in holomictic lakes were often heterotrophic (NCP <0), reflecting a decline in GPP with depth.…”

“…In particular, the suboxic zone shoaled from >14 m depth in February 2016 to <9 m in May 2016. The suboxic zone re-deepened and eventually stabilized around 11 m. While 11 m has been a common depth for the suboxic zone in OTM in recent years 11,12 , in situ data indicate that this is an unstable feature.

Figure 3Depth-integrated ( A ) NCP and ( B ) GPP (blue; above the line) and CR (red; below the line) over time in OTM (based on measurements at 1, 6, and 12 m). ( C ) Integrated lake DO concentrations calculated via trapezoidal integration, and ( D ) suboxic zone depth calculated from DO concentrations.

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“…We tested the reliability of calculating CR and NCP via in situ sensors through multiple concurrent deployments of in situ bottle experiments in different marine lakes 12 . These comparisons included both meromictic (stratified) lakes: Ngermeuangel Lake (abbreviated NLK), Ongeim’l Tketau (OTM aka Jellyfish Lake), and T Lake (TLN); and holomictic (mixed) lakes: Heliofungia Lake (HLO), Mekeald Lake (MLN), Ngeruktabel Lake (NLN), and Uet era Ngchas (ULN).…”

“…‘Marine lakes’ are bodies of seawater surrounded by land that formed as rising sea levels flooded inland basins at the end of the last glaciation 9,10 and are ideal ecosystems in which to examine oxygen dynamics. In particular, marine lakes vary systematically in key aspects—including pronounced gradients in DO and productivity within and across lakes 11,12 —creating natural spatial and temporal gradients in physical, chemical, and biological dynamics. Dozens of marine lakes are located in the Republic of Palau, the most notable being Ongeim’l Tketau (OTM; Fig.…”

“…1A), or Jellyfish Lake, which is typically inhabited by millions of medusa. OTM is a highly-productive meromictic marine lake 12,13 , with a sharp oxycline separating oxygenated surface waters from anoxic bottom water 13 . Permanent stratification is maintained by differences in density between overlying, lower salinity surface waters that receive freshwater runoff, and water of seawater salinity below (Fig.…”