Solar irradiance influences the settlement patterns of mussels

Author:

Uxio Labarta is a research professor at IIM-CSIC (Vigo, Galicia). From the Laboratory of Mussels Ecology and Culture Management, he works on the ecology, physiology, and bioeconomic management of bivalve molluscs. The Lab of Mussels was created in 1987 with Dr. Fernández-Reiriz, and now also integrated by Dr. Fuentes-Santos and the technicians L. Nieto and B. Gonzalez, with the main collaboration of Dr. Alvarez-Salgado of the Marine Geochemistry Lab and PROINSA Mussel Farm.

The life cycle of most benthic marine invertebrates involves a pelagic larval phase that can last for several weeks until individuals are competent to settle and a post-larval sedentary stage. Larval settlement, the process that links larval and sessile stages, is determined by larval production, pre-settlement survival and transport and plays a pivotal role on the population dynamics of these animals. Many studies have addressed the effects of hydrodynamics on larval dispersal and, consequently, on the spatial variability of larval settlement, while the temporal pattern of larval settlement has been mainly attributed to their reproductive cycle.

Mussels are dominant organisms on many rocky shores worldwide, where they play an important ecological role as habitat or prey for a multitude of organisms. In addition, mussel culture has important commercial value. The Galician Rías (NW Iberian coast), located in the northern boundary of the Iberian-Canary Current upwelling system, are characterized by a high mussel production. Indeed Galicia, with more than 250.000 tons/year is the second largest mussel producer in the World behind China. The high amount of seed, c.a. 4700 tons/year, required to support mussel aquaculture in this region led producers to develop suspended collection systems and has motivated an increasing interest on the settlement processes of mussels and the underlying environmental factors^{1 2}. This research has been conducted jointly by the Spanish National Research Council (CSIC) and PROINSA Mussel Farm in order to evaluate and improve the current recruitment and culture strategies.

Marine invertebrates are thought to synchronize their reproductive cycles to produce larvae under optimal temperature and food conditions. It has been stablished that settlement of mussel larvae in Galicia occurs between mid-spring and early autumn. However, after five years (2009-2010, 2012-2014) of weekly monitoring of larval settlement in the Ría de Ares-Betanzos (Galicia), we have identified two different temporal patterns: some yeas the settlement cycle extended from mid-April to October (2009, 2012, 2014), while others reported a delay in the settlement onset until June (2010, 2013). Identifying the environmental factors underlying interannual variability in the onset, length and intensity of larval settlement is crucial to a better understanding of this process.

This work ³ aimed to develop cost-effective models to predict the settlement period and intensity of the mussel M. galloprovincialis in the temperate NW Iberian coast according with meteorological factors. For this purpose we have applied Generalized Additive Models (GAM), which take advantage of the interpretability of linear models and flexibility of nonparamtric techniques, for real and functional data. Taking into account the recent findings about the relationship between solar irradiance and the reproductive schedule of corals, as well as the effect of the wind regime and continental run-off on larval settlement of mussels in our study area, we considered these meteorological factors as explanatory variables

We have seen that the fortnight mean solar irradiance is a good predictor of the probability of larval settlement a month ahead, which allows us to predict the onset and end of the settlement period. Furthermore, our results suggest that the phenology of mussel spawning is mainly driven by the time when solar irradiance reaches a certain threshold (R ≈ 12 MJ/(m²day)). Thus, late winters with high solar irradiances enhance earlier spring spawning events and the subsequent spring settlement peak, while late winters with low solar irradiances delay the onset of the reproductive and settlement cycles.

On the other hand, a functional GAM fit using monthly curves of solar irradiance and continental runoff provided accurate estimators of settlement intensity two weeks in advance. This result highlights the advantages of using functional data, which provide a valuable tool to incorporate all the information obtained by continuous monitoring of meteorological and environmental factors in biological models. This information is particularly valuable in our case because the effects of solar irradiance and continental runoff on seawater temperature and food availability, which drive larval production and survival, are not immediate. Furthermore, as the larval pelagic phase of M. galloprovincialis can last between 10 and 30 days the curves of solar irradiance and continental runoff from 15 to 45 days prior to sampling allowed us to include in the model the influence of these factors on larval production, larval growth, and pre-settlement survival.

This work, in addition to establishing solar radiation as a climate forcing for the production, development and settlement of mussel larvae, provides a useful tool to evaluate and improve the current seed collection strategies in the Galician Rías. Finally it should also be noted that the flexibility of nonparametric models, which let the data talk by themselves, shall allow us to identify any change in the relationships between environmental factors and larval settlement, and consequently our approach can be extended to other areas.