Understanding phytoplankton dynamics in Hvalfjörður : community structure and management of harmful algae

Abstract

Hvalfjörður is a popular shellfish picking spot in Iceland. Mussels are filter feeders on microalgae, some microalgae produce harmful substances (phycotoxins) that accumulate in the mussel meat and can be a deadly threat to human consumers. However, monitoring for toxic algae and phycotoxins levels in the shellfish has not been carried out for several years. This poses a risk to human health as high toxin levels have been measured before in shellfish from the fjord. This study documents the seasonal phytoplankton community structure in Hvalfjörður, with emphasis on toxin forming species and provides recommendations for management of harmful algal blooms (HABs) in Iceland. Sampling for phytoplankton diversity and abundance, primary productivity by chlorophyll a, environmental measurements by a CTD, and nutrient measurements were conducted from April-November 2024 at three locations in the fjord. Phytoplankton samples (n=55) were analysed using a Flow Imaging Microscope and chlorophyll a measured on a fluorometer (n=124). Results of chlorophyll a indicate an autumn bloom with highest primary production in September and October at stations HV5, the middle of the fjord and HV10, inner fjord. The seasonal species diversity of the phytoplankton indicated a dominance of diatoms in spring and autumn and dinoflagellates in summer. Toxigenic species, including the amnesic shellfish toxin forming diatoms, Pseudo-nitzschia spp. and diarrhetic shellfish toxin forming dinoflagellates Dinophysis spp., were present throughout the year in Hvalfjörður posing a risk to the marine food web and people consuming shellfish from the area. To improve HABs management in Iceland, establishing a monitoring program to track toxic phytoplankton species and HABs at recreational picking spots is recommended along with a public awareness campaign about the risk of HABs. Climate change causes ocean temperatures to rise and pH levels to decline leading to acidification. HABs may become more frequent and abundant, species composition changes and toxin production can rise due to climate change. Management strategies need to take these changes into account and future research should include the effect of climate change on HABs, identify high risk areas and seasons for recreational shellfish picking and HABs.

Hvalfjörður er vinsæll staður fyrir tínslu á bláskel, en skortur hefur verið á reglubundinni vöktun á eitruðum þörungum og þörungaeitri þar síðustu ár. Bláskelin síar fæðu sína úr sjónum og fæðan er svifþörungar. Sumar svifþörungategundir mynda þörungaeitur og getur bláskelin safnað í sig þörungaeitri sem er hættulegt heilsu manna. Fyrri rannsóknir hafa sýnt hátt magn þörungaeiturs í skelfiski úr firðinum. Í þessari rannsókn er árstíðabundin þróun samfélaga svifþörunga í Hvalfirði skoðuð, með áherslu á tegundir sem framleiða þörungaeitur. Sýni voru tekin frá apríl til nóvember 2024 á þremur stöðum í firðinum og greind með Flow Imaging Microscope (n=55). Frumframleiðni var metin með mælingum á blaðgrænu a (n=124) en aðrir umhverfisþættir voru mældir með CTD og mælingum á helstu næringarefnum sjávar. Niðurstöðurnar sýna haustblóma með hámarks frumframleiðni í september og október, og ríkjandi tegundir voru kísilþörungar á vorin og haustin, en svipuþörungar yfir sumarið. Eiturmyndandi tegundir, þar á meðal Pseudo-nitzschia spp. og Dinophysis spp., voru til staðar allt árið og gætu því hafa skapað verulega heilsufarsáhættu. Til að draga úr áhættu og bæta viðbragð við skaðlegum þörungablómum er mælt með stofnun vöktunarkerfis á vinsælum tínslustöðum ásamt aukinni fræðslu til almennings. Því er jafnframt mikilvægt að framtíðarstefna Íslands taki mið af áhrifum loftslagsbreytinga á tíðni, útbreiðslu og eiturefnamyndun í svifþörungum á Íslandi.