45th Annual Larval Fish Conference & 13th International Larval Biology Symposium San Diego, California 29 August – 1 September, 2022

INDITU

A new approach for incorporating 15N isotopic data into linear inverse ecosystem models with Markov Chain Monte Carlo sampling.

Oceanographic field programs often use δ15N biogeochemical measurements and in situ rate measurements to investigate nitrogen cycling and planktonic ecosystem structure. However, integrative modeling approaches capable of synthesizing these distinct measurement types are lacking. We develop a novel approach for incorporating δ15N isotopic data into existing Markov Chain Monte Carlo (MCMC) random walk methods for solving linear inverse ecosystem models. We test the ability of this approach to recover food web indices (nitrate uptake, nitrogen fixation, zooplankton trophic level, and secondary production) derived from forward models simulating the planktonic ecosystems of the California Current and Amazon River Plume. We show that the MCMC with δ15N approach typically does a better job of recovering ecosystem structure than the standard MCMC or L2 minimum norm (L2MN) approaches, and also outperforms an L2MN with δ15N approach. Furthermore, we find that the MCMC with δ15N approach is robust to the removal of input equations and hence is well suited to typical pelagic ecosystem studies for which the system is usually vastly under-constrained. Our approach is easily extendable for use with δ13C isotopic measurements or variable carbon:nitrogen stoichiometry

Oral-atrial length and developmental stage composition of Salpa thompsoni collected during RV Tangaroa cruise TAN1810 to the Chatham Rise, New Zealand

Development stage composition (demography) of a Salpa thomsponi population was studied at the Chatham Rise, New Zealand in October and November 2018 using oblique krill net tows (2 mm mesh size, 3.2 m² mouth opening) in the top 200 m of the water column. Three areas with similar oceanographic characteristics were sampled over the course of three weeks. Development stages were defined for blastozooids and oozooids according to classifications from Foxton (1966) and Daponte et al. (2001). Oral-atrial length of salp specimens were measured and blastozooid/oozooid ratios calculated for each sampling event

Salp biology (Salpa thompsoni) at the Chatham Rise, New Zealand

The abundant pelagic tunicate Salpa thompsoni is a major grazer in the Southern Ocean. We report the length-frequency distribution, maturity stage composition, growth, and size-specific diel vertical abundance patterns at one of the northernmost habitats of S. thompsoni (Chatham Rise, east of New Zealand, ~ 44°S 178°E). By observing the in situ growth of distinct size cohorts using integrative krill net tows (upper 200 m, 2 mm mesh size, 3.2 m² mouth opening) and ex situ on-board experiments, relative growth was estimated for small blastozooids to be between 8.8–11.7% d−1 at ambient temperatures of 11 °C. Integrative Bongo tows in the upper 200 m (202 µm mesh size, 0.4 m² mouth opening) showed that S. thompsoni not only have daytime-dependent vertical abundance patterns, but also that these are size-specific, with medium-sized blastozooids and large oozooids contributing most to the elevated values during the night

Physical oceanography during RV Tangaroa cruise TAN1810 to the Chatham Rise, New Zealand

Between 11 and 21 vertical CTD profiles were recorded in the top 300 m of the water column in five areas over the Chatham Rise, New Zealand in October and November 2018. Temperature, salinity, fluorescence, and dissolved oxygen concentrations were measured. The latter two were converted to final values using calibrations

Daytime-dependent abundance of Salpa thomsponi during RV Tangaroa cruise TAN1810 to the Chatham Rise, New Zealand

Daytime-dependent abundance changes of Salpa thomsponi within the top 200 m of the water column were studied at the Chatham Rise, New Zealand in October 2018 using oblique Bongo net tows (202 µm mesh size, 0.4 m² mouth opening). Specimens were identified to life cycle stage (blastozooid, oozooid) and measured for oral-atrial length. Differences in life cycle stage and length-frequency distribution patterns were compared among three consecutive days and nights

Blastozooid growth experiment of Salpa thomsponi during RV Tangaroa cruise TAN1810 to the Chatham Rise, New Zealand

A mature Salpa thompsoni oozooid was collected using a so-called salp-net (1 m diameter ring net with 202 µm mesh fitted with a 30 litre non-filtering cod-end) at the Chatham Rise, New Zealand in October 2018. The growth of new-released blastozooid was studied in an experimental tank on-board for 5 days using image analysis. Growth rates were determined, which fell well within the range of earlier published rates, when temperature was accounted for

Smoothed chlorophyll a profiles during RV Tangaroa cruise TAN1810 to the Chatham Rise, New Zealand

Between 11 and 21 vertical CTD profiles were recorded in the top 300 m of the water column in five areas over the Chatham Rise, New Zealand in October and November 2018. Raw fluorscence data were converted to chlorophyll a concentrations using calibration curves and subsequently smoothed using modelling tools for every m in each area