Carbon dioxide (CO 2 ) is the primary substrate for photosynthesis by the phytoplankton that form the base of the marine food web and mediate biogeochemical cycling of C and nutrient elements. Specific growth rate and elemental composition (C:N:P) were characterized for 7 cosmopolitan coastal and oceanic phytoplankton species (5 diatoms and 2 chlorophytes) using low density, nutrient-replete, semi-continuous culture experiments in which CO 2 was manipulated to 4 levels ranging from post-bloom/glacial maxima (< 290 ppm) to geological maxima levels (> 2900 ppm). Specific growth rates at high CO 2 were from 19 to 60% higher than in low CO 2 treatments in 4 species and 44% lower in 1 species; there was no significant change in 2 species. Higher CO 2 availability also resulted in elevated C:P and N:P molar ratios in Thalassiosira pseudonana (~60 to 90% higher), lower C:P and N:P molar ratios in 3 species (~20 to 50% lower), and no change in 3 species. Carbonate system-driven changes in growth rate did not necessarily result in changes in elemental composition, or vice versa. In a subset of 4 species for which fatty acid composition was examined, elevated CO 2 did not affect the contribution of polyunsaturated fatty acids to total fatty acids significantly. These species show relatively little sensitivity between present day CO 2 and predicted ocean acidification scenarios (year 2100). The results, however, demonstrate that CO 2 availability at environmentally and geologically relevant scales can result in large changes in phytoplankton physiology, with potentially large feedbacks to ocean biogeochemical cycles and ecosystem structure.

Environmental DNA (eDNA) metabarcoding was used to characterize finfish communities in the nearshore estuarine environment. Monthly sampling was conducted June-August 2017 at two sites with structured habitats: a natural rock reef and a shellfish aquaculture farm within the same coastal embayment of Long Island Sound (LIS), CT, United States. Seventeen common and 25 rare finfish taxa were detected using eDNA metabarcoding. Incomplete status of reference sequence databases for finfish species was identified as a methodological challenge. Confidence in molecular identification was improved appreciably through the use of publicly available data obtained from local trawling and seining surveys. Comparison between eDNA metabarcoding and trawling surveys on 6/27/2017, the only day when both data types were available, revealed more finfish species detected by eDNA metabarcoding. The high sensitivity of eDNA metabarcoding detected finfish species rarely observed in traditional surveys and showed the potential for this methodology to augment existing literature for finfish species distribution patterns and invasive species detection. Non-metric multidimensional scaling (NMS) analysis of finfish communities achieved a low-stress, 2D solution, and revealed greater variation between samples collected from different months than samples collected from the two habitats. Similarly, permutational analysis of variance (PERMANOVA) found both month and the interaction term (month × site) significant, with the latter identifying site as significant only in July and August. Different finfish assemblages were significantly associated with each axis, axes representing temporal and spatial variations, respectively. Additionally, polycarbonate and nylon filters were compared to optimize the sampling method; finfish communities retrieved using the two types of filters were statistically indistinguishable by NMS analysis, although the filtration time for nylon filters was shorter. If the objective is to detect rare species, nylon filters are recommended over polycarbonate filters because of higher capture rates of rare taxa. Our study demonstrates the potential for applying eDNA metabarcoding as a stand-alone method to conduct finfish surveys with high sensitivity.

Multi-tiered oyster aquaculture cages may provide habitat for fish assemblages similar to natural structured seafloor. Methods were developed to assess fish assemblages associated with aquaculture gear and boulder habitat using underwater video census combined with environmental DNA (eDNA) metabarcoding. Action cameras were mounted on 3 aquaculture cages at a commercial eastern oyster Crassostrea virginica farm (‘cage’) and among 3 boulders on a natural rock reef (‘boulder’) from June to August 2017 in Long Island Sound, USA. Interval and continuous video recording strategies were tested. During interval recording, cameras collected 8 min video segments hourly from 07:00 to 19:00 h on cages only. Continuous video was also collected for 2-3 h on oyster cages and boulders. Data loggers recorded light intensity and current speed. Seawater was collected for eDNA metabarcoding on the reef and farm. MaxN measurements of fish abundance were calculated in video, and 7 fish species were observed. Black sea bass Centropristis striata, cunner Tautogolabrus adspersus, scup Stenotomus chrysops, and tautog Tautoga onitis were the most abundant species observed in both oyster cage and boulder videos. In continuous video, black sea bass, scup, and tautog were observed more frequently and at higher abundance on the cage farm, while cunner were observed more frequently and at higher abundance on boulders within the rock reef. eDNA metabarcoding detected 42 fish species at the farm and reef. Six species were detected using both methods. Applied in tandem, video recording and eDNA provided a comprehensive approach for describing fish assemblages in difficult to sample structured oyster aquaculture and boulder habitats.