Improving Productivity of Casco Bay Kelp Farms Using Spatiotemporal Analysis of Coastal Nutrient Data

Environmental Data 

We collected new environmental data from 5 locations in Casco Bay during the Spring 2018 and 2019: Peaks Island, Long Island, Clapboard Island, Brothers Island, and Cow Ledge (Figure 1). Mean monthly water temperatures at each site were calculated from continuous measurements made by suspended loggers (Hobo Pendant Temperature/Light Loggers; UA-002-08). Mean monthly salinity and nitrate concentrations were characterized using grab samples collected from 2 m depth with a Niskin bottle. All samples were stored in Whirlpak bags prior to processing. Salinity of the water was assessed using a refractometer (Cole-Parmer RSA-BR90A; 0 – 90%). Nitrate-N was quantified spectrophotometrically using Hach Nitrate TNTplus Low Range Vial Tests and a Hach DR3900 VIS spectrophotometer. 

Kelp Cultivation and Sampling 

Saccharina latissima (sugar kelp) was cultivated at the Brothers Island, Cow Ledge, and Long Island locations in collaboration with partner farmers. The kelp sporelings were produced using techniques described in Redmond et al. (2014) and a density of 6,000 - 8,000 spores per mL in the inoculation water. Outplanted kelp was maintained at 2m depth throughout the cultivation cycle and the kelp longlines at each farm were oriented parallel to the prevailing currents (N-S). Longline density varied between 1 - 5 longlines depending on the site. Spacing between longlines at the same site was ≥ 6m. 

We collected samples from the growing kelp from January until its harvest in mid to late May (Figure 2). Our target sampling frequency was 14 days, but it varied slightly with boat availability and water conditions. During each sampling event, we removed 20 kelp thalli from a randomly-determined location on the kelp longline to be analyzed back at the laboratory. The samples were stored in plastic bags in a cooler during transport and then transferred to a refrigerator until they could be processed. 

Assessing Morphology and Growth

In the laboratory, we measured and recorded the length, three width measurements (at 25%, 50%, and 75% of total length), and total wet weight of each blade (Figure 3). Length, diameter, and total wet weight were recorded for each stipe. Notes regarding the presence/absence of sorus tissue, degree of fouling, and general condition of the specimen were also recorded.

The meristematic portion (site of active growth) of 5 randomly selected thalli from the sample were also prepared for carbon and nitrogen analysis. These samples were rinsed with deionized water and dried in an oven at 50°C for 24 hours. Once dry, each sample was ground and homogenized into a fine powder. A known quantity, 2 - 5mg, of this powder was encapsulated to prepare three replicates. The samples were shipped to the Stable Isotope Facility at UC Davis. At this facility the samples were analyzed for total nitrogen and total carbon using a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer. The sample’s preliminary nitrogen and carbon content was measured relative to reference gases analyzed with each sample. This measurement of total nitrogen or carbon content was divided by the sample weight to obtain %C or %N for the dry kelp. 

Statistical and Spatial Analysis

All data were examined for assumptions of normality and heterogeneity, and outliers were removed. Multivariate analyses of variance (MANOVA) were used to compare the effect of the environmental factors on morphology of the cultivated kelp. If significant effects (p ≤ 0.05) were detected, then separate, one-way analyses of variance were used to analyze each dependent variable. Then Tukey’s Honest Significant Difference tests and Pearson R correlations were measured between ambient nitrate and S. latissima morphology.

Publicly available environmental data were also downloaded from regional datasets and combined with the new environmental data. Salinity and sea surface temperature data for Spring 2018 and 2019 were downloaded from 12 locations routinely sampled by volunteers for Friends of Casco Bay (FOCB). Sea surface temperature data was also downloaded from the Northwest Atlantic Marine Ecoregional Assessment conducted by The Nature Conservancy (TNC). Inverse distance weighting (IDW) was used to spatially interpolate between the environmental data. The interpolation procedure produced separate raster layers for each environmental variable with smoothed predictions between individual sampling points. The IDW interpolation was performed in QGIS (3.10.1).